In order to understand the microsolvation of LiI and CsI in water and provide information about the dependence of solvation processes on different ions, we investigated the LiI(H2O)n– and CsI(H2O)n– (n = 0–6) clusters using photoelectron spectroscopy. The structures of these clusters and their corresponding neutrals were investigated with ab initio calculations and confirmed by comparing with the photoelectron spectroscopy experiments. Our studies show that the structural evolutions of LiI(H2O)n and CsI(H2O)n clusters are very different. The Li–I distance in LiI(H2O)n– increases abruptly at n = 3, whereas the abrupt elongation of the Li–I distance in neutral LiI(H2O)n occurs at n = 5. In contrast to the LiI(H2O)n– clusters, the Cs–I distance in CsI(H2O)n– increases significantly at n = 3, reaches a maximum at n = 4, and decreases again as n increases further. There is no abrupt change of the Cs–I distance in neutral CsI(H2O)n as n increases from 0 to 6. Water molecules interact strongly with the Li ion; consequently, water molecule(s) can insert within the Li+–I– ion pair. In contrast, five or six water molecules are not enough to induce obvious separation of the Cs+–I– ion pair since the Cs–water interaction is relatively weak compared to the Li–water interaction. Our work has shown that the structural variation and microsolvation in MI(H2O)n clusters are determined by the delicate balance between ion–ion, ion–water, and water–water interactions, which may have significant implications for the general understanding of salt effects in water solution.

The GWGW method has become the state-of-the-art approach for the first-principles description of the electronic quasi-particle band structure in crystalline solids. Most of the existing codes rely on pseudopotentials in which only valence electrons are treated explicitly. The pseudopotential method can be problematic for systems with localized dd- or ff-electrons, even for ground-state density-functional theory (DFT) calculations. The situation can become more severe in GWGW calculations, because pseudo-wavefunctions are used in the computation of the self-energy and the core–valence interaction is approximated at the DFT level. In this work, we present the package FHI-gap, an all-electron GWGW implementation based on the full-potential linearized augmented planewave plus local orbital (LAPW) method. The FHI-gap code can handle core, semicore, and valence states on the same footing, which allows for a correct treatment of core–valence interaction. Moreover, it does not rely on any pseudopotential or frozen-core approximation. It is, therefore, able to handle a wide range of materials, irrespective of their composition. Test calculations demonstrate the convergence behavior of the results with respect to various cut-off parameters. These include the size of the basis set that is used to expand the products of Kohn–Sham wavefunctions, the number of k points for the Brillouin zone integration, the number of frequency points for the integration over the imaginary axis, and the number of unoccupied states. At present, FHI-gap is linked to the WIEN2k code, and an implementation into the exciting code is in progress.

The 3d–4f exchange interaction plays an important role in many lanthanide based molecular magnetic materials such as single-molecule magnets and magnetic refrigerants. In this work, we study the 3d–4f magnetic exchange interactions in a series of Cu(II)–Gd(III) (3d9–4f7) dinuclear complexes based on the numerical atomic basis-norm-conserving pseudopotential method and density functional theory plus the Hubbard U correction approach (DFT+U). We obtain improved description of the 4f electrons by including the semicore 5s5p states in the valence part of the Gd-pseudopotential. The Hubbard U correction is employed to treat the strongly correlated Cu-3d and Gd-4f electrons, which significantly improve the agreement of the predicted exchange constants, J, with experiment, indicating the importance of accurate description of the local Coulomb correlation. The high efficiency of the DFT+U approach enables us to perform calculations with molecular crystals, which in general improve the agreement between theory and experiment, achieving a mean absolute error smaller than 2 cm–1. In addition, through analyzing the physical effects of U, we identify two magnetic exchange pathways. One is ferromagnetic and involves an interaction between the Cu-3d, O-2p (bridge ligand), and the majority-spin Gd-5d orbitals. The other one is antiferromagnetic and involves Cu-3d, O-2p, and the empty minority-spin Gd-4f orbitals, which is suppressed by the planar Cu–O–O–Gd structure. This study demonstrates the accuracy of the DFT+U method for evaluating the 3d–4f exchange interactions, provides a better understanding of the exchange mechanism in the Cu(II)–Gd(III) complexes, and paves the way for exploiting the magnetic properties of the 3d–4f compounds containing lanthanides other than Gd.

Molybdenum and tungsten dichalcogenides, MX2 (M = Mo and W; X = S and Se), characterized by their quasi-two-dimensional layered structure, have attracted intensive interest due to their intriguing physical and chemical properties. In this work, quasi-particle electronic properties of these materials are investigated by many-body perturbation theory in the GW approximation, currently the most accurate first-principles approach for electronic band structure of extended systems. It is found that the fundamental band gaps of all of these materials can be well described by the GW approach, and the calculated density of states from GW quasi-particle band energies agree very well with photoemission spectroscopy data. Ionization potentials of these materials are also studied by combining the slab model using density functional theory and GW correction. On the basis of our theoretical findings, we predict that none of the materials in MX2 (M = Zr, Hf, Mo, and W; X = S and Se) in their bulk form can be directly used as the photocatalyst for overall photosplitting of water because their VBM and CBM energies do not match the redox potentials of water oxidation and reduction, which, however, can be changed by forming nanostructures, especially for MoS2.

Accurate evaluation of the total energy difference between different spin states in molecular magnetic systems is currently a great challenge in theoretical chemistry. In this work we assess the performance of the density functional theory plus the Hubbard U (DFT+U) approach for the first-principles description of the high spin-low spin (HS-LS) splitting and the exchange coupling constant, corresponding to the intra- and interatomic spin interactions, respectively. The former is investigated using a set of mononuclear ion complexes with different HS-LS splitting, including seven spin-crossover (SCO) compounds, while the latter is investigated in a series of binuclear copper complexes covering both ferromagnetic and antiferromagnetic interactions. We find that the DFT+U approach can reproduce experimental data as accurately as the hybrid functionals approach but with much lower computational efforts. We further analyze the effect of U in terms of spin density on magnetic centers, and we find that the main effect of the U correction can be attributed to the enhanced localization of magnetic orbitals. Even taking the uncertainty related to the determination of U into account, we think the DFT+U approach is an efficient and predictive first-principles method for the SCO phenomenon and interatomic magnetic interactions.

Alkaline tantalates, ATaO3 (A = Li, Na, and K), have attracted a lot of interest in recent years due to their interesting photocatalytic properties and their photocatalytic activity is influenced by a lot of factors, making them ideal model systems for in-depth theoretical investigation. In this work, electronic band structures of alkaline tantalates are investigated based on first-principles many-body perturbation theory in the GW approximation. The band gaps of NaTaO3 and KTaO3 from the GW approach agree very well with experiment; on the other hand, the band gap of LiTaO3 from GW is significantly larger than the experimental values. A strong dependence on crystal structures is observed in LiTaO3, whose band gap in the cubic and rhombohedral structure differs by more than 1.5 eV. Combined with the phenomenological ionic model, it is found that both the Madelung potential and the bandwidth can have strong influences on the band gap. By comparing the structure dependence of LiTaO3 and NaTaO3, it is concluded that the intra-TaO6-octahedron distortion has stronger effects on electronic band structures than the inter-TaO6-octahedron distortion. Possible causes underlying the discrepancy between GW and experiment for LiTaO3 are also analyzed.

Electronic band structure is one of the most important intrinsic properties of a material, and is in particular crucial in electronic, photo-electronic and photo-catalytic applications. Kohn-Sham Density-functional theory (KS-DFT) within currently available local or semi-local approximations to the exchange-correlation energy functional is problematic for the description of electronic band structure. Many-body perturbation theory based on Green’s function (GF) provides a rigorous framework to describe excited-state properties of materials. The central ingredient of the GF-based many-body perturbation theory is the exchangecorrelation self-energy, which accounts for all nonclassical electron-electron interaction effects beyond the Hartree theory, and formally can be obtained by solving a set of complicated integro-differential equations, named Hedin’s equations. The GW approximation, in which the self-energy is simply a product of Green’s function and the screened Coulomb interaction (W), is currently the most accurate first-principles approach to describe electronic band structure properties of extended systems. Compared to KS-DFT, the computational efforts required for GW calculations are much larger. Various numerical techniques or approximations have been developed to apply GW for realistic systems. In this paper, we give an overview of the theory of first-principles Green’s function approach in the GW approximation and review the state of the art for the implementation of GW in different representations and with different treatment of the frequency dependence. It is hoped that further methodological developments will be inspired by this work so that the approach can be applied to more complicated and scientifically more interesting systems.