Many ionic liquids containing the dicynamide anion (DCA–, formula N(CN)2–) exhibit hypergolic ignition when exposed to the common oxidizer nitric acid. However, the ignition delay is often about 10 times longer than the desired 5 ms for rocket applications, so that improvements are desired. Experiments in the past decade have suggested both a mechanism for the early reaction steps and also that additives such as decaborane can reduce the ignition delay. The mechanisms for reactions of nitric acid with both DCA– and protonated DCAH are considered here, using accurate wave function methods. Complexation of DCA– or DCAH with borane clusters B10H14 or B9H14– is found to modify these mechanisms slightly by changing the nature of some of the intermediate saddle points and by small reductions in the reaction barriers.

The importance of dispersion forces in water clusters is examined using the effective fragment potential (EFP) method. Since the original EFP1 water potential does not include dispersion, a dispersion correction to the EFP1 potential (EFP1-D) was derived and implemented. The addition of dispersion to the EFP1 potential yields improved geometries for water clusters that contain 2–6 molecules. The importance of the odd E7 contribution to the dispersion energy is investigated. The E7 dispersion term is repulsive for all of the water clusters studied here and can have a magnitude that is as large as half of the E6 value. The E7 term therefore contributes to larger intermolecular distances for the optimized geometries. Inclusion of many-body effects and/or higher order terms may be necessary to further improve dispersion energies and optimized geometries.

When group III metals are deposited onto the Si(100)-2 × 1 reconstructed surface they are observed to self-assemble into chains of atoms that are one atom high by one atom wide. To better understand this one-dimensional island growth, ab initio electronic structure calculations on the structures of Al atoms on silicon clusters have been performed. Natural orbital occupation numbers show that these systems display significant diradical character, suggesting that a multireference method is needed. A multiconfiguration self-consistent field (MCSCF) calculation with a 6-31G(d) basis set and effective core potentials was used to optimize geometries. The surface integrated molecular orbital molecular mechanics embedded cluster method was used to take the surface chemistry into account, as well as the structure of an extended surface region. Potential energy surfaces for binding of Al adatoms and Al−Al dimers on the surface were determined, and the former was used to obtain a preliminary assessment of the surface diffusion of adatoms. Hessians were calculated to characterize stationary points, and improved treatment of dynamic electron correlation was accomplished using multireference second order perturbation theory (MRMP2) single-point energy calculations. Results from the MRMP2//MCSCF embedded cluster calculations are compared with those from QM-only cluster calculations, embedded cluster unrestricted density functional theory calculations, and previous Car−Parrinello DFT studies.

The analytic gradient for the Coulomb, polarization, exchange-repulsion, and dispersion terms of the fully integrated effective fragment molecular orbital (EFMO) method is derived and the implementation is discussed. The derivation of the EFMO analytic gradient is more complicated than that for the effective fragment potential (EFP) gradient, because the geometry of each EFP fragment is flexible (not rigid) in the EFMO approach. The accuracy of the gradient is demonstrated by comparing the EFMO analytic gradient with the numeric gradient for several systems, and by assessing the energy conservation during an EFMO NVE ensemble molecular dynamics simulation of water molecules. In addition to facilitating accurate EFMO geometry optimizations, this allows calculations with flexible EFP fragments to be performed.

The analytic first derivative with respect to nuclear coordinates is formulated and implemented in the framework of the three-body fragment molecular orbital (FMO) method. The gradient has been derived and implemented for restricted second-order Møller–Plesset perturbation theory, as well as for both restricted and unrestricted Hartree–Fock and density functional theory. The importance of the three-body fully analytic gradient is illustrated through the failure of the two-body FMO method during molecular dynamics simulations of a small water cluster. The parallel implementation of the fragment molecular orbital method, its parallel efficiency, and its scalability on the Blue Gene/Q architecture up to 262 144 CPU cores are also discussed.

The dispersion interaction energy may be expressed as a sum over R–n terms, with n ≥ 6. Most implementations of the dispersion interaction in model potentials are terminated at n = 6. Those implementations that do include higher order contributions commonly only include even power terms, despite the fact that odd power terms can be important. Because the effective fragment potential (EFP) method contains no empirically fitted parameters, the EFP method provides a useful vehicle for examining the importance of the leading R–7 odd power term in the dispersion expansion. To fully evaluate the importance of the R–7 contribution to the dispersion energy, it is important to have analytic energy first derivatives for all terms. In the present work, the gradients of the term E7 ∼ R–7 are derived analytically, implemented in the GAMESS software package, and evaluated relative to other terms in the dispersion expansion and relative to the total EFP interaction energy. Periodic boundary conditions in the minimum image convention are also implemented. A more accurate dispersion energy contribution can now be obtained during molecular dynamics simulations.

The aldol reaction catalyzed by an amine-substituted mesoporous silica nanoparticle (amine-MSN) surface was investigated using a large molecular cluster model (Si392O958C6NH361) combined with the surface integrated molecular orbital/molecular mechanics (SIMOMM) and fragment molecular orbital (FMO) methods. Three distinct pathways for the carbinolamine formation, the first step of the amine-catalyzed aldol reaction, are proposed and investigated in order to elucidate the role of the silanol environment on the catalytic capability of the amine-MSN material. The computational study reveals that the most likely mechanism involves the silanol groups actively participating in the reaction, forming and breaking covalent bonds in the carbinolamine step. Therefore, the active participation of MSN silanol groups in the reaction mechanism leads to a significant reduction in the overall energy barrier for the carbinolamine formation. In addition, a comparison between the findings using a minimal cluster model and the Si392O958C6NH361 cluster suggests that the use of larger models is important when heterogeneous catalysis problems are the target.

Protonation of the anion in an ionic liquid plays a key role in the hypergolic reaction between ionic liquids and oxidizers such as white fuming nitric acid. To investigate the influence of the cation on the protonation reaction, the deprotonation energy of a set of cations has been calculated at the MP2 level of theory. Specifically, guanidinium, dimethyltriazanium, triethylamine, N-ethyl-N-methyl-pyrrolidinium, N-ethyl-pyridinium, 1,4-dimethyl-1,2,4-triazolium, 1-ethyl-4-methyl-1,2,4-triazolium, and 1-butyl-4-methyl-1,2,4-triazolium were studied. In addition, the net proton transfer energies from the cations to a set of previously studied anions was calculated, demonstrating an inverse correlation between the net proton transfer energy and the likelihood that the cation/anion combination will react hypergolically with white fuming nitric acid. It is suggested that this correlation occurs due to a balance between the energy released by the proton transfer and the rate of proton transfer as determined by the ionicity of the ionic liquid.

A Ce atom reaction with ethylene was carried out in a laser-vaporization metal cluster beam source. Ce(C2H2) formed by hydrogen elimination from ethylene was investigated by mass-analyzed threshold ionization (MATI) spectroscopy, isotopic substitutions, and relativistic quantum chemical computations. The theoretical calculations include a scalar relativistic correction, dynamic electron correlation, and spin–orbit coupling. The MATI spectrum exhibits two nearly identical band systems separated by 128 cm–1. The separation is not affected by deuteration. The two-band systems are attributed to spin–orbit splitting and the vibrational bands to the symmetric metal–ligand stretching and in-plane carbon–hydrogen bending excitations. The spin–orbit splitting arises from interactions of a pair of nearly degenerate triplets and a pair of nearly degenerate singlets. The organolanthanide complex is a metallacyclopropene in C2v symmetry. The low-energy valence electron configurations of the neutral and ion species are Ce 4f16s1 and Ce 4f1, respectively. The remaining two electrons that are associated with the isolated Ce atom or ion are spin paired in a molecular orbital that is a bonding combination between a 5d Ce orbital and a π* antibonding orbital of acetylene.

The computational efficiency and energy-to-solution of several applications using the GAMESS quantum chemistry suite of codes is evaluated for 32-bit and 64-bit ARM-based computers, and compared to an x86 machine. The x86 system completes all benchmark computations more quickly than either ARM system and is the best choice to minimize time to solution. The ARM64 and ARM32 computational performances are similar to each other for Hartree–Fock and density functional theory energy calculations. However, for memory-intensive second-order perturbation theory energy and gradient computations the lower ARM32 read/write memory bandwidth results in computation times as much as 86% longer than on the ARM64 system. The ARM32 system is more energy efficient than the x86 and ARM64 CPUs for all benchmarked methods, while the ARM64 CPU is more energy efficient than the x86 CPU for some core counts and molecular sizes.

The surface affinity of the nitrate ion in aqueous clusters is investigated with a variety of theoretical methods. A sampling of structures in which the nitrate ion is solvated by 32 water molecules is optimized using second order Møller–Plesset perturbation theory (MP2). Four of these MP2 optimized structures are used as starting points for fully ab initio molecular dynamics simulations at the dispersion corrected restricted Hartree–Fock (RHF-D) level of theory. The nitrate ion solvated by 16, 32, and 64 water molecules is also investigated with umbrella sampling molecular dynamics simulations using QM/MM methodology, where the nitrate ion is modeled with MP2 and the water molecules are described using either the non-empirical effective fragment potential (EFP) or the empirical TIP5P potential. The turning point between surface and interior solvation of the nitrate ion is predicted to lie around a cluster size of 64 water molecules.

The modeling of dispersion interactions in density functional theory (DFT) is commonly performed using an energy correction that involves empirically fitted parameters for all atom pairs of the system investigated. In this study, the first-principles-derived dispersion energy from the effective fragment potential (EFP) method is implemented for the density functional theory (DFT-D(EFP)) and Hartree–Fock (HF-D(EFP)) energies. Overall, DFT-D(EFP) performs similarly to the semiempirical DFT-D corrections for the test cases investigated in this work. HF-D(EFP) tends to underestimate binding energies and overestimate intermolecular equilibrium distances, relative to coupled cluster theory, most likely due to incomplete accounting for electron correlation. Overall, this first-principles dispersion correction yields results that are in good agreement with coupled-cluster calculations at a low computational cost.

The local correlation “cluster-in-molecule” (CIM) method is combined with the fragment molecular orbital (FMO) method, providing a flexible, massively parallel, and near-linear scaling approach to the calculation of electron correlation energies for large molecular systems. Although the computational scaling of the CIM algorithm is already formally linear, previous knowledge of the Hartree–Fock (HF) reference wave function and subsequent localized orbitals is required; therefore, extending the CIM method to arbitrarily large systems requires the aid of low-scaling/linear-scaling approaches to HF and orbital localization. Through fragmentation, the combined FMO-CIM method linearizes the scaling, with respect to system size, of the HF reference and orbital localization calculations, achieving near-linear scaling at both the reference and electron correlation levels. For the 20-residue alanine α helix, the preliminary implementation of the FMO-CIM method captures 99.6% of the MP2 correlation energy, requiring 21% of the MP2 wall time. The new method is also applied to solvated adamantine to illustrate the multilevel capability of the FMO-CIM method.

The direct simulation of the solid–liquid water interface with the effective fragment potential (EFP) via the constant enthalpy and pressure (NPH) ensemble was used to estimate the melting temperature (Tm) of ice-Ih. Initial configurations and velocities, taken from equilibrated constant pressure and temperature (NPT) simulations at P = 1 atm and T = 305 K, 325 K and 399 K, respectively, yielded corresponding Tm values of 378 ± 16 K, 382 ± 14 K and 384 ± 15 K. These estimates are consistently higher than experiment, albeit to the same degree as previously reported estimates using density functional theory (DFT)-based Born–Oppenheimer simulations with the Becke-Lee–Yang–Parr functional plus dispersion corrections (BLYP-D).

The R–7 term (E7) in the dispersion expansion is developed in the framework of the general effective fragment potential (EFP2) method, formulated with the dynamic anisotropic Cartesian polarizability tensors over the imaginary frequency range. The E7 formulation is presented in terms of both the total molecular polarizability and the localized molecular orbital (LMO) contributions. An origin transformation from the center of mass to the LMO centroids is incorporated for the computation of the LMO dipole–quadrupole polarizability. The two forms considered for the damping function for the R–7 dispersion interaction, the overlap-based and Tang–Toennies damping functions, are extensions of the existing damping functions for the R–6 term in the dispersion expansion. The R–7 dispersion interaction is highly orientation dependent: it can be either attractive or repulsive, and its magnitude can change substantially as the relative orientation of two interacting molecules changes. Although the R–7 dispersion energy rotationally averages to zero, it may be significant for systems in which rotational averaging does not occur, such as rotationally rigid molecular systems as in molecular solids or constrained surface reactions.

Increasingly, modern computer systems comprise a multicore general-purpose processor augmented with a number of special purpose devices or accelerators connected via an external interface such as a PCI bus. The NVIDIA Kepler Graphical Processing Unit (GPU) and the Intel Phi are two examples of such accelerators. Accelerators offer peak performances that can be well above those of the host processor. How to exploit this heterogeneous environment for legacy application codes is not, however, straightforward. This paper considers how matrix operations in typical quantum chemical calculations can be migrated to the GPU and Phi systems. Double precision general matrix multiply operations are endemic in electronic structure calculations, especially methods that include electron correlation, such as density functional theory, second order perturbation theory, and coupled cluster theory. The use of approaches that automatically determine whether to use the host or an accelerator, based on problem size, is explored, with computations that are occurring on the accelerator and/or the host. For data-transfers over PCI-e, the GPU provides the best overall performance for data sizes up to 4096 MB with consistent upload and download rates between 5–5.6 GB/s and 5.4–6.3 GB/s, respectively. The GPU outperforms the Phi for both square and nonsquare matrix multiplications.

The surface affinity of the hydronium ion in water is investigated with umbrella sampling and classical molecular dynamics simulations, in which the system is described with the effective fragment potential (EFP). The solvated hydronium ion is also explored using second order perturbation theory for the hydronium ion and the empirical TIP5P potential for the waters. Umbrella sampling is used to analyze the surface affinity of the hydronium ion, varying the number of solvent water molecules from 32 to 256. Umbrella sampling with the EFP method predicts the hydronium ion to most probably lie about halfway between the center and edge of the water cluster, independent of the cluster size. Umbrella sampling using MP2 for the hydronium ion and TIP5P for the solvating waters predicts that the solvated proton most probably lies about 0.5–2.0 Å from the edge of the water cluster independent of the cluster size.

Anionic water clusters are generally considered to be extremely challenging to model using fragmentation approaches due to the diffuse nature of the excess electron distribution. The local correlation coupled cluster (CC) framework cluster-in-molecule (CIM) approach combined with the completely renormalized CR-CC(2,3) method [abbreviated CIM/CR-CC(2,3)] is shown to be a viable alternative for computing the vertical electron binding energies (VEBE). CIM/CR-CC(2,3) with the threshold parameter ζ set to 0.001, as a trade-off between accuracy and computational cost, demonstrates the reliability of predicting the VEBE, with an average percentage error of ∼15% compared to the full ab initio calculation at the same level of theory. The errors are predominantly from the electron correlation energy. The CIM/CR-CC(2,3) approach provides the ease of a black-box type calculation with few threshold parameters to manipulate. The cluster sizes that can be studied by high-level ab initio methods are significantly increased in comparison with full CC calculations. Therefore, the VEBE computed by the CIM/CR-CC(2,3) method can be used as benchmarks for testing model potential approaches in small-to-intermediate-sized water clusters.

The fully analytic energy gradient has been developed and implemented for the restricted open-shell Hartree–Fock (ROHF) method based on the fragment molecular orbital (FMO) theory for systems that have multiple open-shell molecules. The accuracy of the analytic ROHF energy gradient is compared with the corresponding numerical gradient, illustrating the accuracy of the analytic gradient. The ROHF analytic gradient is used to perform molecular dynamics simulations of an unusual open-shell system, liquid oxygen, and mixtures of oxygen and nitrogen. These molecular dynamics simulations provide some insight about how triplet oxygen molecules interact with each other. Timings reveal that the method can calculate the energy gradient for a system containing 4000 atoms in only 6 h. Therefore, it is concluded that the FMO-ROHF method will be useful for investigating systems with multiple open shells.

This work presents a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane, using the ab initio multiple spawning (AIMS) program that has been interfaced with the General Atomic and Molecular Electronic Structure System (GAMESS) quantum chemistry package for on-the-fly electronic structure evaluation. The interface strategy is discussed, and the capabilities of the combined programs are demonstrated with a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane. Energies, gradients, and nonadiabatic coupling matrix elements were obtained with the state-averaged complete active space self-consistent field method, as implemented in GAMESS. The influence of initial vibrational excitation on the outcome of the photoinduced isomerization is explored. Increased vibrational excitation in the CNNC torsional mode shortens the excited state lifetime. Depending on the degree of vibrational excitation, the excited state lifetime varies from ∼60–200 fs. These short lifetimes are in agreement with time-resolved photoionization mass spectroscopy experiments.

The design and development of scientific software components to provide an interface to the effective fragment potential (EFP) methods are reported. Multiscale modeling of physical and chemical phenomena demands the merging of software packages developed by research groups in significantly different fields. Componentization offers an efficient way to realize new high performance scientific methods by combining the best models available in different software packages without a need for package readaptation after the initial componentization is complete. The EFP method is an efficient electronic structure theory based model potential that is suitable for predictive modeling of intermolecular interactions in large molecular systems, such as liquids, proteins, atmospheric aerosols, and nanoparticles, with an accuracy that is comparable to that of correlated ab initio methods. The developed components make the EFP functionality accessible for any scientific component-aware software package. The performance of the component is demonstrated on a protein interaction model, and its accuracy is compared with results obtained with coupled cluster methods.

A new coupled cluster singles and doubles with triples correction, CCSD(T), algorithm is presented. The new algorithm is implemented in object oriented C++, has a low memory footprint, fast execution time, low I/O overhead, and a flexible storage backend with the ability to use either distributed memory or a file system for storage. The algorithm is demonstrated to work well on single workstations, a small cluster, and a high-end Cray computer. With the new implementation, a CCSD(T) calculation with several hundred basis functions and a few dozen occupied orbitals can run in under a day on a single workstation. The algorithm has also been implemented for graphical processing unit (GPU) architecture, giving a modest improvement. Benchmarks are provided for both CPU and GPU hardware.

In this work, the effective fragment potential (EFP) method is fully integrated (FI) into the fragment molecular orbital (FMO) method to produce an effective fragment molecular orbital (EFMO) method that is able to account for all of the fundamental types of both bonded and intermolecular interactions, including many-body effects, in an accurate and efficient manner. The accuracy of the method is tested and compared to both the standard FMO method as well as to fully ab initio methods. It is shown that the FIEFMO method provides significant reductions in error while at the same time reducing the computational cost associated with standard FMO calculations by up to 96%.

•The EFP method performs as well as MP2 and coupled cluster, much better than DFT-D methods.•When DFT-D performs reasonably well, it is because of cancellation of errors.•When HF-D performs reasonably well, there is little or no error cancellation.•Many body effects in water arise primarily from induction.•Many body effects in argon arise primarily from dispersion.Water and argon hexamers are examined using correlated wavefunction methods, the effective fragment potential (EFP) method, and Hartree–Fock (HF) theory and several density functional theory (DFT) functionals. The HF and DFT methods have been employed both with and without dispersion corrections. The computationally inexpensive EFP method captures the high-level coupled cluster binding energy and relative isomer energy predictions very well for both types of hexamer, much better than the DFT methods (DFT-D), even those that include dispersion corrections. Interestingly, the dispersion corrected HF method (HF-D) does very well. When the DFT-D methods perform reasonably, they do so because of a fortuitous off-setting cancellation of errors two-body and many-body contributions to the binding energy. The HF-D method does not rely on such error cancellation, while the EFP method captures both two-body and many-body contributions to the water hexamer very well. The many body contribution to the argon cluster are small and are most likely due to dispersion.Graphical abstract

Excited-state enol to keto tautomerization of 7-hydroxy-4-methylcoumarin (C456) with three water molecules (C456:3H2O), is theoretically investigated using time-dependent density functional theory (TDDFT) combined with the polarizable continuum model and 200 waters explicitly modeled with the effective fragment potential. The tautomerization of C456 in the presence of three water molecules is accompanied by an asynchronous quadruple hydrogen atom transfer reaction from the enol to the keto tautomer in the excited state. TDDFT with the PBE0 functional and the DH(d,p) basis set is used to calculate the excited-state reaction barrier height, absorption (excitation), and fluorescence (de-excitation) energies. These results are compared with the available experimental and theoretical data. In contrast to previous work, it is predicted here that the coumarin 456 system undergoes a hydrogen atom transfer, not a proton transfer. The calculated reaction barrier of the first excited state of C456:3H2O with 200 water molecules is found to be −0.23 kcal/mol without zero-point energy (−5.07 kcal/mol with zero point energy, i.e., the activation energy).

Solvent effects on the electronic spectra of formamide and trans-N-methylacetamide are studied using four different levels of theory: singly excited configuration interaction (CIS), equations of motion coupled-cluster theory with singles and doubles (EOM-CCSD), completely renormalized coupled-cluster theory with singles and doubles with perturbative triple excitations (CR-EOM-CCSD(T)), and time-dependent density functional theory (TDDFT), employing small clusters of water molecules. The simulated electronic spectrum is obtained via molecular dynamics simulations with 100 waters modeled with the effective fragment potential method and exhibits a blue-shift and red-shift, respectively, for the n → π* and πnb → π* vertical excitation energies, in good agreement with the experimental electronic spectra of amides.

Fragment molecular orbital molecular dynamics (FMO-MD) with periodic boundary conditions is performed on liquid water using the analytic energy gradient, the electrostatic potential point charge approximation, and the electrostatic dimer approximation. Compared to previous FMO-MD simulations of water that used an approximate energy gradient, inclusion of the response terms to provide a fully analytic energy gradient results in better energy conservation in the NVE ensemble for liquid water. An FMO-MD simulation that includes the fully analytic energy gradient and two body corrections (FMO2) gives improved energy conservation compared with a previously calculated FMO-MD simulation with an approximate energy gradient and including up to three body corrections (FMO3).

In this article, a new multithreaded Hartree–Fock CPU/GPU method is presented which utilizes automatically generated code and modern C++ techniques to achieve a significant improvement in memory usage and computer time. In particular, the newly implemented Rys Quadrature and Fock Matrix algorithms, implemented as a stand-alone C++ library, with C and Fortran bindings, provides up to 40% improvement over the traditional Fortran Rys Quadrature. The C++ GPU HF code provides approximately a factor of 17.5 improvement over the corresponding C++ CPU code.

Benchmark timings are presented for the fragment molecular orbital method on a Blue Gene/P computer. Algorithmic modifications that lead to enhanced performance on the Blue Gene/P architecture include strategies for the storage of fragment density matrices by process subgroups in the global address space. The computation of the atomic forces for a system with more than 3000 atoms and 44 000 basis functions, using second order perturbation theory and an augmented and polarized double-ζ basis set, takes ∼7 min on 131 072 cores.

Two electronic structure methods, the fragment molecular orbital (FMO) and systematic molecular fragmentation (SMF) methods, that are based on fragmenting a large molecular system into smaller, more computationally tractable components (fragments), are presented and compared with fully ab initio results for the predicted binding energies of water clusters. It is demonstrated that, even when explicit three-body effects are included (especially necessary for water clusters due to their complex hydrogen-bonded networks) both methods present viable, computationally efficient alternatives to fully ab initio quantum chemistry.

A series of twelve platinum(II) complexes of the form (N^N^N)PtX have been synthesized and characterized where N^N^N is 1,3-bis(2-pyridylimino)isoindolate ligands (BPI) or BPI ligands whose aryl moieties are substituted with tert-butyl, nitro, alkoxy, iodo or chloro groups, and X is a chloride, fluoride, cyano, acetate, phenyl or 4-(dimethylamino)phenyl ligand. All complexes display at least one irreversible oxidation and two reversible reduction waves at potentials dependent on the position and the electron donating or withdrawing nature of both X and the substituted N^N^N ligand. Broad room temperature phosphorescence ranging in energy from 594 to 680 nm was observed from the complexes, with quantum efficiencies ranging from 0.01 to 0.05. The efficiency of emission is dictated largely by nonradiative processes since the rate constants for nonradiative deactivation [(1.1–100) × 105 s−1] show greater variation than those for radiative decay [(0.57–4.0) × 04 s−1]. Nonradiative deactivation for compounds with X = Cl follow the energy gap law, i.e. the nonradiative rate constants increase exponentially with decreasing emission energy. Deactivation of the excited state appears to be strongly influenced by a non-planar distortion of the BPI ligand.

An investigation of the structure and dynamics of the high-energy ionic liquid, 1-hydroxyethyl-4-amino-1,2,4-triazolium nitrate (HEATN), was undertaken. Both experimental and computational methods were employed to understand the fundamental properties, characteristics, and behavior of HEATN. The charge separation, according to the electrostatic potential derived charges, was assessed. The MP2 (second-order perturbation theory) geometry optimizations find six dimer and five tetramer structures and allow one to see the significant highly hydrogen bonded network predicted within the HEATN system. Due to the prohibitive scaling of ab initio methods, the fragment molecular orbital (FMO) method was employed and assessed for feasibility with highly energetic ionic liquids using HEATN as a model system. The FMO method was found to adequately treat the HEATN ionic liquid system as evidenced by the small relative error obtained. The experimental studies involved the investigation of the solvation dynamics of the HEATN system via the coumarin 153 (C153) probe at five different temperatures. The rotational dynamics through the HEATN liquid were also measured using C153. Comparisons with previously studied imidazolium and phosphonium ionic liquids show surprising similarity. To the authors’ knowledge, this is the first experimental study of solvation dynamics in a triazolium-based ionic liquid.

The anharmonicity of Li+–(H2)n (n = 1, 2, and 3) complexes is studied using the vibrational self-consistent field (VSCF) approach. The H–H stretching frequency shifts of Li+–(H2)n complexes are calculated with the coupled-cluster method including all single and double excitations with perturbative triples (CCSD(T)) level of theory with the cc-pVTZ basis set. The calculated IR active H–H stretching frequency in Li+–H2, Li+–(H2)2 and Li+–(H2)3 is red-shifted by 121, 109, and 96–99 cm–1, respectively, relative to that of isolated H2. The calculated red shifts and their trends are in good agreement with the available experimental data.

The ability to perform geometry optimizations on large molecular systems is desirable for both closed- and open-shell species. In this work, the restricted open-shell Hartree–Fock (ROHF) gradients for the fragment molecular orbital (FMO) method are presented. The accuracy of the gradients is tested, and the ability of the method to reproduce adiabatic excitation energies is also investigated. Timing comparisons between the FMO method and full ab initio calculations are also performed, demonstrating the efficiency of the FMO method in modeling large open-shell systems.

The effects of solvents on electronic spectra can be treated efficiently by combining an accurate quantum mechanical (QM) method for the solute with an efficient and accurate method for the solvent molecules. One of the most sophisticated approaches for treating solvent effects is the effective fragment potential (EFP) method. The EFP method has been interfaced with several QM methods, including configuration interaction, time-dependent density functional theory, multiconfigurational methods, and equations-of-motion coupled cluster methods. These combined QM–EFP methods provide a range of efficient and accurate methods for studying the impact of solvents on electronic excited states. An energy decomposition analysis in terms of physically meaningful components is presented in order to analyze these solvent effects. Several factors that must be considered when one investigates solvent effects on electronic spectra are discussed, and several examples are presented.

Neutral and anionic 13-atom aluminum clusters are studied with high-level, fully ab initio methods: second-order perturbation theory (MP2) and coupled cluster theory with singles, doubles, and perturbative triples (CCSD(T)). Energies and vibrational frequencies are reported for icosahedral and decahedral isomers, and are compared with density functional theory results. At the MP2 level of theory, with all of the basis sets employed, the icosahedral structure is energetically favored over the decahedral structure for both the neutral and anionic Al13 clusters. Hessian calculations imply that only the icosahedral structures are potential energy minima. The CCSD(T)/aug-cc-pVTZ adiabatic electron affinity of Al13 is found to be 3.57 eV, in excellent agreement with experiment.

Theoretical calculations and solid-state NMR have been used to determine the conformation, relative energies, and behavior of organic functional groups covalently bound within the pores of mesoporous silica nanoparticles (MSNs). The calculations were performed using the ReaxFF reactive force field for model surfaces consisting of a four-layer silica slab with one or two functional groups: N-(2-aminoethyl)-3-aminopropyl- (AAP), N-[N-(2-aminoethyl)-2-aminoethyl]-3-aminopropyl- (AEP), or 3-cyanopropyl- (CP). The results indicate that the AAP and AEP groups exist primarily in the prone orientation, while CP can almost equally occupy both the prone and upright orientations in CP-MSN. This is in agreement with the solid-state 13C NMR experiments, which suggest that the AAP and AEP functionalities remain rigid on the NMR time scale (in this case sub-millisecond), whereas the CP substituent executes faster motions. These conformations are most likely governed by the hydrogen bonds between the amine moieties of the functional groups and the silanol groups on the silica surface. ReaxFF can be used to study a system that requires a large-scale model, such as the surface of an organo-functionalized heterogeneous catalyst, with higher accuracy than the conventional MM and at a lower computational cost than ab initio quantum mechanical calculations.

The anharmonicity of weakly bound complexes is studied using the vibrational self-consistent field (VSCF) approach for a series of metal cation dihydrogen (M+−H2) complexes. The H−H stretching frequency shifts of M+−H2 (M+ = Li+, Na+, B+, and Al+) complexes are calculated with the coupled-cluster method including all single and double excitations with perturbative triples (CCSD(T)) level of theory with the cc-pVTZ basis set. The calculated H−H stretching frequency of Li+−H2, B+−H2, Na+−H2, and Al+−H2 is red-shifted by 121, 202, 74, and 62 cm-1, respectively, relative to that of unbound H2. The calculated red shifts and their trends are in good agreement with the available experimental and previously calculated data. Insight into the observed trends is provided by symmetry adapted perturbation theory (SAPT).

The photoisomerization process of 1,2-diphenylethylene (stilbene) is investigated using the spin-flip density functional theory (SFDFT), which has recently been shown to be a promising approach for locating conical intersection (CI) points (Minezawa, N.; Gordon, M. S. J. Phys. Chem. A2009, 113, 12749). The SFDFT method gives valuable insight into twisted stilbene to which the linear response time-dependent DFT approach cannot be applied. In contrast to the previous SFDFT study of ethylene, a distinct twisted minimum is found for stilbene. The optimized structure has a sizable pyramidalization angle and strong ionic character, indicating that a purely twisted geometry is not a true minimum. In addition, the SFDFT approach can successfully locate two CI points: the twisted-pyramidalized CI that is similar to the ethylene counterpart and another CI that possibly lies on the cyclization pathway of cis-stilbene. The mechanisms of the cis–trans isomerization reaction are discussed on the basis of the two-dimensional potential energy surface along the twisting and pyramidalization angles.

The combined time-dependent density functional theory effective fragment potential method (TDDFT/EFP1) is applied to a study of the solvent-induced shift of the lowest singlet π → π* charge-transfer excited state of p-nitroaniline (pNA) from the gas to the condensed phase in water. Molecular dynamics simulations of pNA with 150 EFP1 water molecules are used to model the condensed-phase and generate a simulated spectrum of the lowest singlet charge-transfer excitation. The TDDFT/EFP1 method successfully reproduces the experimental condensed-phase π → π* vertical excitation energy and solvent-induced red shift of pNA in water. The largest contribution to the red shift comes from Coulomb interactions, between pNA and water, and solute relaxation. The solvent shift contributions reflect the increase in zwitterionic character of pNA upon solvation.

In this work, the dynamical nucleation theory (DNT) model using the ab initio based effective fragment potential (EFP) is implemented for evaluating the evaporation rate constant and molecular properties of molecular clusters. Predicting the nucleation rates of aerosol particles in different chemical environments is a key step toward understanding the dynamics of complex aerosol chemistry. Therefore, molecular scale models of nanoclusters are required to understand the macroscopic nucleation process. On the basis of variational transition state theory, DNT provides an efficient approach to predict nucleation kinetics. While most DNT Monte Carlo simulations use analytic potentials to model critical sized clusters, or use ab initio potentials to model very small clusters, the DNTEFP Monte Carlo method presented here can treat up to critical sized clusters using the effective fragment potential (EFP), a rigorous nonempirical intermolecular model potential based on ab initio electronic structure theory calculations, improvable in a systematic manner. The DNTEFP method is applied to study the evaporation rates, energetics, and structure factors of multicomponent clusters containing water and isoprene. The most probable topology of the transition state characterizing the evaporation of one water molecule from a water hexamer at 243 K is predicted to be a conformer that contains six hydrogen bonds, with a topology that corresponds to two water molecules stacked on top of a quadrangular (H2O)4 cluster. For the water hexamer in the presence of isoprene, an increase in the cluster size and a 3-fold increase in the evaporation rate are predicted relative to the reaction in which one water molecule evaporates from a water hexamer cluster.

Highly accurate excitation spectra are predicted for the low-lying n−π* and π−π* states of uracil for both the gas phase and in water employing the complete active space self-consistent field (CASSCF) and multiconfigurational quasidegenerate perturbation theory (MCQDPT) methods. Implementation of the effective fragment potential (EFP) solvent method with CASSCF and MCQDPT enables the prediction of highly accurate solvated spectra, along with a direct interpretation of solvent shifts in terms of intermolecular interactions between solvent and solute. Solvent shifts of the n−π* and π−π* excited states arise mainly from a change in the electrostatic interaction between solvent and solute upon photoexcitation. Polarization (induction) interactions contribute about 0.1 eV to the solvent-shifted excitation. The blue shift of the n−π* state is found to be 0.43 eV and the red shift of the π−π* state is found to be −0.26 eV. Furthermore, the spectra show that in solution the π−π* state is 0.4 eV lower in energy than the n−π* state.

Benzannulation of aromatic molecules is often used to red-shift absorption and emission bands of organic and inorganic, molecular, and polymeric materials; however, in some cases, either red or blue shifts are observed, depending on the site of benzannulation. A series of five platinum(II) complexes of the form (N∧N∧N)PtCl are reported here that illustrate this phenomenon, where N∧N∧N represents the tridentate monoanionic ligands 2,5-bis(2-pyridylimino)3,4-diethylpyrrolate (1), 1,3-bis(2-pyridylimino)isoindolate (2), 1,3-bis(2-pyridylimino)benz(f)isoindolate (3), 1,3-bis(2-pyridylimino)benz(e)isoindolate (4), and 1,3-bis(1-isoquinolylimino) isoindolate (5). For this series of molecules, either a blue shift (2 and 3) or a red shift (4 and 5) in absorption and emission maxima, relative to their respective nonbenzannulated compounds, was observed that depends on the site of benzannulation. Experimental data and first principles calculations suggest that a similar HOMO energy level and a destabilized or stabilized LUMO with benzannulation is responsible for the observed trends. A rationale for LUMO stabilization/destabilization is presented using simple molecular orbital theory. This explanation is expanded to describe other molecules with this unusual behavior.

An implementation is presented of an uncontracted Rys quadrature algorithm for electron repulsion integrals, including up to g functions on graphical processing units (GPUs). The general GPU programming model, the challenges associated with implementing the Rys quadrature on these highly parallel emerging architectures, and a new approach to implementing the quadrature are outlined. The performance of the implementation is evaluated for single and double precision on two different types of GPU devices. The performance obtained is on par with the matrix−vector routine from the CUDA basic linear algebra subroutines (CUBLAS) library.

Performing accurate calculations on large molecular systems is desirable for closed- and open-shell systems. In this work, the fragment molecular orbital method is extended to open-shell systems and implemented in the GAMESS (General Atomic and Molecular Electronic Structure System) program package. The accuracy of the method is tested, and the ability to reproduce reaction enthalpies is demonstrated. These tests also demonstrate its utility in providing an efficient means to model large open-shell systems.

The rate constants for the gas-phase reactions in the silicon carbide chemical vapor deposition of methyltrichlorosilane (Ge, Y. B.; Gordon, M. S.; Battaglia, F.; Fox, R. O. J. Phys. Chem. A 2007, 111, 1462.) were calculated. Transition state theory was applied to the reactions with a well-defined transition state; canonical variational transition state theory was applied to the barrierless reactions by finding the generalized transition state with the maximum Gibbs free energy along the reaction path. Geometry optimizations were carried out with second-order perturbation theory (MP2) and the cc-pVDZ basis set. The partition functions were calculated within the harmonic oscillator and rigid rotor approximations. The final potential energy surfaces were obtained using the left-eigenstate coupled-cluster theory, CR-CC(2,3) with the cc-pVTZ basis set. The high-pressure approximation was applied to the unimolecular reactions. The predicted rate constants for more than 50 reactions were compared with the experimental ones at various temperatures and pressures; the deviations are generally less than 1 order of magnitude. Theory is found to be in reasonable agreement with the experiments.

The solvation of alanine is investigated, with a focus on adding a sufficient number of discrete water molecules to determine the first solvation shell for both the nonionized (N) and zwitterionic (Z) forms to converge the enthalpy of solvation and the enthalpy difference for the two forms of alanine. Monte Carlo sampling was employed using the generalized effective fragment potential (EFP) method to determine the global minimum of both conformers, with the number of EFP water molecules ranging from 32−49. A subset of sampled geometries were optimized with second-order perturbation theory (MP2) using the 6-31++G(d,p) basis set. Single point energies were calculated at these geometries using the polarizable continuum model (PCM). The predicted 298.15 K enthalpy of solvation ranges for MP2/6-31++G(d,p) and MP2+PCM//MP2/6-31++G(d,p) are 10.0−13.2 kcal/mol and 10.1−12.6 kcal/mol, respectively.

Structural properties of large NO3−·(H2O)n (n = 15−500) clusters are studied by Monte Carlo simulations using effective fragment potentials (EFPs) and by classical molecular dynamics simulations using a polarizable empirical force field. The simulation results are analyzed with a focus on the description of hydrogen bonding and solvation in the clusters. In addition, a comparison between the electronic structure based EFP and the classical force field description of the 32 water cluster system is presented. The EFP simulations, which focused on the cases of n = 15 and 32, show an internal, fully solvated structure and a “surface adsorbed” structure for the 32 water cluster at 300 K, with the latter configuration being more probable. The internal solvated structure and the “surface adsorbed” structure differ considerably in their hydrogen bonding coordination numbers. The force field based simulations agree qualitatively with these results, and the local geometry of NO3− and solvation at the surface-adsorbed site in the force field simulations are similar to those predicted using EFPs. Differences and similarities between the description of hydrogen bonding of the anion in the two approaches are discussed. Extensive classical force field based simulations at 250 K predict that long time scale stability of “internal” NO3−, which is characteristic of extended bulk aqueous interfaces, emerges only for n > 300. Ab initio Møller−Plesset perturbation theory is used to test the geometries of selected surface and interior anions for n = 32, and the results are compared to the EFP and MD simulations. Qualitatively, all approaches agree that surface structures are preferred over the interior structures for clusters of this size. The relatively large aqueous clusters of NO3− studied here are of comparable size to clusters that lead to new particle formation in air. Nitrate ions on the surface of such clusters may have significantly different photochemistry than the internal species. The possible implications of surface-adsorbed nitrate ions for atmospheric chemistry are discussed.

Conical intersections (CIs) of ethylene have been successfully determined using spin-flip density functional theory (SFDFT) combined with a penalty-constrained optimization method. We present in detail three structures, twisted-pyramidalized, hydrogen-migrated, and ethylidene CIs. In contrast to the linear response time-dependent density functional theory, which predicts a purely twisted geometry without pyramidalization as the S1 global minimum, SFDFT gives a pyramidalized structure. Therefore, this is the first correct optimization of CI points of twisted ethylene by the DFT method. The calculated energies and geometries are in good agreement with those obtained by the multireference configuration interaction (MR-CI) method and the multistate formulation of second-order multireference perturbation theory (MS-CASPT2).

An ab initio study of the addition of successive water molecules to the amino acid l-alanine in both the nonionized (N) and zwitterionic (Z) forms are presented. The main focus is the number of waters needed to stabilize the Z form and how the solvent affects conformational preference. The solvent is modeled by ab initio electronic structure theory, the EFP (effective fragment potential) model, and the isotropic dielectric PCM (polarizable continuum method) bulk solvation techniques. The EFP discrete solvation model is used with a Monte Carlo algorithm to sample the configuration space to find the global minimum. Bridging structures are predicted to be the lowest energy Z minima after 3−5 discrete waters are included in the calculations, depending on the level of theory. Second-order perturbation theory and PCM stabilize the Z structures by ∼3−6 and 7 kcal/mol, respectively, relative to the N global minimum through the addition of up to 8 waters. Subsequently, the contributions of each are ∼1 kcal/mol relative to the N global minimum. The presence of 32 waters appears to be close to converging the N−Z enthalpy difference, ΔHN−Z.

The recently developed restricted open-shell, size extensive, left eigenstate, completely renormalized (CR), coupled-cluster (CC) singles (S), doubles (D), and noniterative triples (T) approach, termed CR-CC(2,3) and abbreviated in this paper as ROCCL, is compared with the unrestricted CCSD(T) [UCCSD(T)] and multireference second-order perturbation theory (MRMP2) methods to assess the accuracy of the calculated potential energy surfaces (PESs) of eight single bond-breaking reactions of open-shell species that consist of C, H, Si, and Cl; these types of reactions are interesting because they account for part of the gas-phase chemistry in the silicon carbide chemical vapor deposition. The full configuration interaction (FCI) and multireference configuration interaction with Davidson quadruples correction [MRCI(Q)] methods are used as benchmark methods to evaluate the accuracy of the ROCCL, UCCSD(T), and MRMP2 PESs. The ROCCL PESs are found to be in reasonable agreement with the corresponding FCI or MRCI(Q) PESs in the entire region R = 1−3Re for all of the studied bond-breaking reactions. The ROCCL PESs have smaller nonparallelity error (NPE) than the UCCSD(T) ones and are comparable to those obtained with MRMP2. Both the ROCCL and UCCSD(T) PESs have significantly smaller reaction energy errors (REE) than the MRMP2 ones. Finally, an efficient strategy is proposed to estimate the ROCCL/cc-pVTZ PESs using an additivity approximation for basis set effects and correlation corrections.

A series of twelve platinum(II) complexes of the form (N^N^N)PtX have been synthesized and characterized where N^N^N is 1,3-bis(2-pyridylimino)isoindolate ligands (BPI) or BPI ligands whose aryl moieties are substituted with tert-butyl, nitro, alkoxy, iodo or chloro groups, and X is a chloride, fluoride, cyano, acetate, phenyl or 4-(dimethylamino)phenyl ligand. All complexes display at least one irreversible oxidation and two reversible reduction waves at potentials dependent on the position and the electron donating or withdrawing nature of both X and the substituted N^N^N ligand. Broad room temperature phosphorescence ranging in energy from 594 to 680 nm was observed from the complexes, with quantum efficiencies ranging from 0.01 to 0.05. The efficiency of emission is dictated largely by nonradiative processes since the rate constants for nonradiative deactivation [(1.1–100) × 105 s−1] show greater variation than those for radiative decay [(0.57–4.0) × 04 s−1]. Nonradiative deactivation for compounds with X = Cl follow the energy gap law, i.e. the nonradiative rate constants increase exponentially with decreasing emission energy. Deactivation of the excited state appears to be strongly influenced by a non-planar distortion of the BPI ligand.

Two electronic structure methods, the fragment molecular orbital (FMO) and systematic molecular fragmentation (SMF) methods, that are based on fragmenting a large molecular system into smaller, more computationally tractable components (fragments), are presented and compared with fully ab initio results for the predicted binding energies of water clusters. It is demonstrated that, even when explicit three-body effects are included (especially necessary for water clusters due to their complex hydrogen-bonded networks) both methods present viable, computationally efficient alternatives to fully ab initio quantum chemistry.

The surface affinity of the nitrate ion in aqueous clusters is investigated with a variety of theoretical methods. A sampling of structures in which the nitrate ion is solvated by 32 water molecules is optimized using second order Møller–Plesset perturbation theory (MP2). Four of these MP2 optimized structures are used as starting points for fully ab initio molecular dynamics simulations at the dispersion corrected restricted Hartree–Fock (RHF-D) level of theory. The nitrate ion solvated by 16, 32, and 64 water molecules is also investigated with umbrella sampling molecular dynamics simulations using QM/MM methodology, where the nitrate ion is modeled with MP2 and the water molecules are described using either the non-empirical effective fragment potential (EFP) or the empirical TIP5P potential. The turning point between surface and interior solvation of the nitrate ion is predicted to lie around a cluster size of 64 water molecules.