A least-squares commutator in the iterative subspace (LCIIS) approach is explored for accelerating self-consistent field (SCF) calculations. LCIIS is similar to direct inversion of the iterative subspace (DIIS) methods in that the next iterate of the density matrix is obtained as a linear combination of past iterates. However, whereas DIIS methods find the linear combination by minimizing a sum of error vectors, LCIIS minimizes the Frobenius norm of the commutator between the density matrix and the Fock matrix. This minimization leads to a quartic problem that can be solved iteratively through a constrained Newton’s method. The relationship between LCIIS and DIIS is discussed. Numerical experiments suggest that LCIIS leads to faster convergence than other SCF convergence accelerating methods in a statistically significant sense, and in a number of cases LCIIS leads to stable SCF solutions that are not found by other methods. The computational cost involved in solving the quartic minimization problem is small compared to the typical cost of SCF iterations and the approach is easily integrated into existing codes. LCIIS can therefore serve as a powerful addition to SCF convergence accelerating methods in computational quantum chemistry packages.

A process for analyzing log files collected from open-ended learning environments is developed and tested on a virtual lab problem involving reaction stoichiometry. The process utilizes a set of visualization tools that, by grouping student actions in a hierarchical manner, helps experts make sense of the linear list of student actions recorded in a raw log file. Such analysis of an initial set of log files is then used to develop a rule-based system that can automatically classify the problem-solving strategies being engaged in by the students. The strategies assigned by the resulting rule-based system compare well with strategy codes assigned by experts.

Field-induced fluorescence quenching of poly(p-phenylene vinylene) (PPV) oligomers due to nonradiative relaxation through free electron–hole pair (FEHP) states is modeled using singles configuration interaction computations with the intermediate neglect of differential overlap Hamiltonian. The computations find FEHP states with energies that drop linearly with applied field and undergo avoided crossings with the fluorescent state. The coupling between the FEHP and fluorescent state, computed for multiple FEHP states on a variety of oligomer lengths, is found to depend primarily on the field strength required for the state to cross the fluorescent state. The rate of decay to these dark FEHP states is then calculated from Marcus theory, which is modified to take into account dielectric in addition to other bulk measurement considerations. The results predict that individual molecules go from being emissive to fully quenched over a small range of applied field strengths. Phenomenological introduction of inhomogeneous broadening for the energies of the FEHP states leads to a more gradual dependence on applied field. The fluorescence quenching mechanism considered here is found to be important for applied fields above about 1 MV cm–1, which is similar in magnitude to those present in light-emitting diodes.

The advanced placement (AP) chemistry course has recently undergone a major course redesign, as specified in a new curriculum framework. This commentary reflects on the community process used to author and review the curriculum framework, along with the intellectual framework of big ideas and science practices that guided the redesign. This contribution is part of a special issue on teaching introductory chemistry in the context of the advanced placement chemistry course redesign.Keywords: Curriculum; First-Year Undergraduate/General; High School/Introductory Chemistry; Inquiry-Based/Discovery Learning; Public Understanding/Outreach; Student-Centered Learning; Testing/Assessment;

Two suggestions for instruction in chemical equilibrium are presented, along with the evidence that supports these suggestions. The first is to use diagrams to connect chemical reactions to the effects of reactions on concentrations. The second is the use of the majority and minority species (M&M) strategy to analyze chemical equilibrium systems. Two studies are presented in support of these suggestions.Keywords: Chemical Education Research; Equilibrium; First-Year Undergraduate/General; Problem Solving/Decision Making;

The fluorescence of the SKC-513 ((E)-N-(9-(4-(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)phenyl)-6-(butyl(3-sulfopropyl)amino)-3H-xanthen-3-ylidene)-N-(3-sulfopropyl)butan-1-aminium) dye is shown experimentally to have high sensitivity to binding of the K+ ion. Computations are used to explore the potential origins of this sensitivity and to make some suggestions regarding structural improvements. In the absence of K+, excitation is to two nearly degenerate states, a neutral (N) excited state with a high oscillator strength, and a charge-transfer (CT) state with a lower oscillator strength. Binding of K+ destabilizes the CT state, raising its energy far above the N state. The increase in fluorescence quantum yield upon binding of K+ is attributed to the increased energy of the CT state suppressing a nonradiative pathway mediated by the CT state. The near degeneracy of the N and CT excited states can be understood by considering SKC-513 as a reduced symmetry version of a parent molecule with 3-fold symmetry. Computations show that acceptor–donor substituents can be used to alter the relative energies of the N and CT state, whereas a methylene spacer between the heterocycle and phenylene groups can be used to increase the coupling between these states. These modifications provide synthetic handles with which to optimize the dye for K+ detection.

A collection of online activities emphasizes the design and interpretation of experiments.

The use of molecular similarity to develop reliable low-cost quantum mechanical models for use in quantum mechanical/molecular mechanical simulations of chemical reactions is explored, using the H + HF → H2 + F collinear reaction as a test case. The approach first generates detailed quantum chemical data for the reaction center in geometries and electrostatic environments that span those expected to arise during the molecular dynamics simulations. For each geometry and environment, both high- and low-level ab initio calculations are performed. A model is then developed to predict the high-level results using only inputs generated from the low-level theory. The inputs used here are based on principal component analysis of the low-level distributed multipoles, and the model is a simple linear regression. The distributed multipoles are monopoles, dipoles, and quadrupoles at each atomic center, and they summarize the electronic distribution in a manner that is comparable across basis set. The error in the model is dominated by extrapolation from small to large basis sets, with extrapolation from uncorrelated to correlated methods contributing much less error. A single regression can be used to make predictions for a range of reaction-center geometries and environments. For the trial collinear reaction, separate regressions were developed for the transition region and the entrance and exit channels. These models can predict the results of CCSD(T)/cc-pVTZ computations from HF/3-21G distributed multipoles, with an average error for the reaction energy profile of 0.69 kcal/mol.

Light-driven molecular motors may be useful for nanotechnology applications. The possibility of building such a motor based on the tolane framework is explored here. In the ground electronic state of tolane, the barrier to internal rotation is comparable to room temperature thermal energies, kBT. The barrier increases substantially in the excited state, causing the molecule to planarize after absorption of a photon. This tendency to planarize may be converted into unidirectional rotational motion by placing chiral substituents on the phenyl rings. A potential advantage of this class of motors is that they may undergo rapid, nanosecond scale rotation. Computational design of appropriate substituents was done using semiempirical quantum chemical methods, SAM1 for the ground electronic state coupled to INDO for the excitation energy. The torsional surfaces of the best candidate were then generated using ab initio DFT methods, which confirm that the molecule should undergo unidirectional rotation upon photoexcitation. The results provide a proof of principle for this class of motors; however, two aspects of the final candidate are nonideal. First, although the design goal was to use steric interactions between substituents to induce the rotation, decomposition of the interaction energy suggests attractive interactions play a role. Solvent interactions may interfere with these attractive interactions. Second, TDDFT calculations suggest that interactions between excited states lower the rotational driving force in the excited state.

Fluorogenic dyes such as thiazole orange (TO) and malachite green have been used in live cellular imaging due to their low quantum yield in solution but large fluorescence enhancements when bound to cellular nucleic acids or to a specific surface-expressed protein partner. Better understanding of the structure−property relationships that establish this fluorogenic behavior could benefit the design of improved dyes. In TO the fluorogenic properties are related to twisting of the dye, following electronic excitation in solution, from an emissive planar structure to a nonemissive twisted structure. Herein we develop a computational approach to identify electron acceptor/donor substitution patterns that impart desirable properties to the dye, such as inducing spectral shifts while maintaining an excited-state torsional surface that will lead to fluorogenic behavior. Additivity of substituent effects, on properties such as spectral shifts and excited-state torsional barriers, is tested and found to be sufficiently accurate that it can be used to identify promising dye candidates. Although additivity suggests an underlying linearity in the substituent effects, additional simplifications stemming from linearity could not be identified. The approach is tested on TO, considering seven different substituents at seven substitution positions, to identify fluorogenic dyes that will span a range of wavelengths. Additivity allows quantum chemical calculations on singly substituted molecules (49 molecules) to be used to make estimates for all substitution patterns (nearly 106 molecules).