Yoshiumi Kawamura

A hybrid approach combining energy density analysis with the interaction energy decomposition method

We propose a new analysis technique for characterizing molecular interactions that combines an energy decomposition scheme, such as the Kitaura–Morokuma decomposition method, with energy density analysis, which partitions the total energy of the system into atomic contributions. The combined scheme, termed Interaction‐EDA, enables us to estimate the local contribution of interaction energy components, such as electrostatic, exchange, polarization, and charge transfer. The evaluation of the local interaction energy is rather important in large systems. For a numerical assessment, the Interaction‐EDA method is applied to the process of CO adsorption on Si(100) − (2 × 1) surface.

Energy density analysis of cluster size dependence of surface-molecule interactions: H2, C2H2, C2H4, and CO adsorption onto Si(100)-(2×1) surface

Adsorption of H2, C2H2, C2H4, and CO onto a Si(100)-(2x1) surface has been treated theoretically using Si(12n - 3)H(8n + 4) (n = 1-4) clusters. The energy density analysis (EDA) proposed by Nakai has been adopted to examine surface-molecule interactions for different cluster sizes. EDA results for the largest model cluster Si45H36 have shown that the adsorption-induced energy density variation in Si atoms decays with distance from the adsorption site. Analysis of this decay, which can be carried out using the EDA technique, is important because it enables verification of the reliability of the model cluster used. In the cases of H2, C2H2, C2H4, and CO adsorption onto the Si(100)-(2x1) surface, it is found that at least a Si21H20 cluster is necessary to treat the surface-molecule interaction with chemical accuracy.

π*–σ* Hyperconjugation mechanism on the rotational barrier of the methyl group (II): 1- and 2-methylnaphthalenes in the S0, S1, C0, and A1 states

Abstract Internal rotation of the methyl group in 1- and 2-methylnaphthalenes has been investigated by the ab initio theory. The rotational barriers in the S 0 and S 1 states calculated by the Hartree–Fock and configuration–interaction with single-excitation operator methods are in reasonable agreement with experimental values. The variations of the rotational barriers by excitation (S 0 →S 1 ), ionization (S 0 →C 0 ), and electron attachment (S 0 →A 1 ) are shown to be directly connected with the stability of the HOMO and/or LUMO by the first-order treatment. In the HOMO and LUMO, a new type of orbital interaction named π*–σ* hyperconjugation appears and determines their stability. The interpretation based on the π*–σ* hyperconjugation can consistently and comprehensively explain the barrier variations.

Energy density analysis of internal methyl rotations in halogenated toluenes

We have recently proposed an energy density analysis (EDA) that partitions the total energy of a molecular system into atomic energy densities. In this study, the EDA was applied to internal methyl rotations of o- and m-halogenated toluenes. For toluene and m-halogenated toluenes, the energy density changes of the ortho-carbons are significant for the rotational barrier height. For o-fluorotoluene, the in-plane hydrogen of the methyl group and fluorine forms a hydrogen bond, decreasing the barrier height. It is shown that the EDA technique is a very useful and powerful tool for investigating chemical and physical phenomena.

π*–σ* hyperconjugation mechanism on the rotational barrier of the methyl group (III): Methyl-azabenzenes in the ground, excited, and anionic states

We theoretically investigate the internal rotations of the methyl group in methyl-azabenzenes, such as o- and m-methylpyridines, 2-methylpyrazine, 4-methylpyrimidine, 4-methylpyridadine, and 4-methyl-1,2,3-triazine in the ground, excited, and anionic states. The calculated rotational barriers reproduce well the experimental data. Orbital pictures are given for the barrier changes by excitation and electron attachment. An idea of π*–σ* hyperconjugation is applied for a comprehensive interpretation of the barrier changes. A correlation is found between the rotational barriers and the splitting of the lowest and next-lowest unoccupied molecular orbitals.

Structure and Ionic Conductivity of Li2S–P2S5 Glass Electrolytes Simulated with First-Principles Molecular Dynamics

Lithium thiophosphate-based materials are attractive as solid electrolytes in all-solid-state lithium batteries because glass or glass-ceramic structures of these materials are associated with very high conductivity. In this work, we modeled lithium thiophosphates with amorphous structures and investigated Li+ mobilities by using molecular dynamics calculations based on density functional theory (DFT-MD). The structures of xLi2S-(100 - x)P2S5 (x = 67, 70, 75, and 80) were created by randomly identifying appropriate compositions of Li+, PS43-, P2S74-, and S2- and then annealing them with DFT-MD calculations. Calculated relative stabilities of the amorphous structures with x = 67, 70, and 75 relative to crystals with the same compositions were 0.04, 0.12, and 0.16 kJ/g, respectively. The implication is that these amorphous structures are metastable. There was good agreement between calculated and experimental structure factors determined from X-ray scattering. The differences between the structure factors of amorphous structures were small, except for the first sharp diffraction peak, which was affected by the environment between Li and S atoms. Li+ diffusion coefficients obtained from DFT-MD calculations at various temperatures for picosecond simulation times were on the order of 10-3 - 10-5 Angstrom2/ps. Ionic conductivities evaluated by the Nernst-Einstein relationship at 298.15 K were on the order of 10-5 S/cm. The ionic conductivity of the amorphous structure with x = 75 was the highest among the amorphous structures because there was a balance between the number density and diffusibility of Li+. The simulations also suggested that isolated S atoms suppress Li+ migration.