Shinji Tsuneyuki

First-principles study of hydrogen bond symmetrization of phase D under high pressure

Abstract We have investigated the physical and structural properties of phase D (MgSi2O6H2) at high pressure by means of a first-principles calculation method. Phase D is important as a dense hydrous magnesium silicate with high stability under pressure, and as one of the most likely candidates for a water reservoir in the Earth’s lower mantle. The calculated compression behavior of phase D is in very good agreement with experimental results. We found a distinct but continuous change from asymmetric to symmetric hydrogen bonding in phase D at 40 GPa. This pressure-induced hydrogen bond symmetrization has a significant effect on the compression behavior of phase D. The bulk modulus increases by about 20% with this structural change. This behavior of pressure-induced hydrogen bond symmetrization is very similar to that previously reported by us for δ-AlOOH. The transition is reversible and second-order, and thus the high-pressure state is probably unquenchable.

Quantum distribution of protons in solid molecular hydrogen at megabar pressures

Solid hydrogen, a simple system consisting only of protons and electrons, exhibits a variety of structural phase transitions at high pressures. Experimental studies based on static compression up to about 230 GPa revealed three relevant phases of solid molecular hydrogen: phase I (high-temperature, low-pressure phase), phase II (low-temperature phase) and phase III (high-pressure phase). Spectroscopic data suggest that symmetry breaking, possibly related to orientational ordering, accompanies the transition into phases II and III. The boundaries dividing the three phases exhibit a strong isotope effect, indicating that the quantum-mechanical properties of hydrogen nuclei are important. Here we report the quantum distributions of protons in the three phases of solid hydrogen, obtained by a first-principles path-integral molecular dynamics method. We show that quantum fluctuations of protons effectively hinder molecular rotation—that is, a quantum localization occurs. The obtained crystal structures have entirely different symmetries from those predicted by the conventional simulations which treat protons classically.

First-principles study of CO bonding to Pt(111) : validity of the Blyholder model

Abstract The chemisorption of CO molecules on Pt(111) is studied by first-principles calculations based on the local density functional formalism with a slab model to represent the extended metal surface. We have developed a population analysis scheme which is applicable to calculations with plane wave basis sets. By applying the scheme to CO Pt (111) , the 4σ and 1π orbitals of CO are found to be completely filled, showing that they do not play a role in the bonding of CO to the surface. On the other hand, the calculated populations of the 5σ and 2π orbitals indicate that there is substantial 5σ donation and 2π backdonation, supporting the Blyholder model of CO chemisorption, the validity of which has recently been questioned. These results also suggest that a molecular-orbital based picture such as the Blyholder model is appropriate for describing simply the chemisorption bond, in spite of the fact that the molecular orbitals of CO rehybridize with each other upon adsorption, as has been shown recently by X-ray emission spectroscopy.

Impact of rattlers on thermal conductivity of a thermoelectric clathrate: a first-principles study.

We investigate the role of rattling guest atoms on the lattice thermal conductivity of a type-I clathrate Ba_{8}Ga_{16}Ge_{30} by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause tenfold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining the unusually low thermal conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone.

Band effect on the charge exchange process at solid surfaces

The finite band effect of target states on the charge exchange process at solid surfaces is elucidated on the basis of the semiclassical time-dependent Anderson Hamiltonian. The width and the density of states of the surface band have much influence on the final charge-transfer probability after ion or atom scattering at surfaces both in the case of resonance tunneling and in the case of energy-level crossing. The band effect is explained mainly by electron diffusion from the target atom to the extended surface region. The formation of a surface molecule is also important in the case of resonance tunneling.

Anharmonic force constants extracted from first-principles molecular dynamics: applications to heat transfer simulations

A systematic method to calculate anharmonic force constants of crystals is presented. The method employs the direct-method approach, where anharmonic force constants are extracted from the trajectory of first-principles molecular dynamics simulations at high temperature. The method is applied to Si where accurate cubic and quartic force constants are obtained. We observe that higher-order correction is crucial to obtain accurate force constants from the trajectory with large atomic displacements. The calculated harmonic and anharmonic force constants are, then, combined with the Boltzmann transport equation (BTE) and non-equilibrium molecular dynamics (NEMD) methods in calculating the thermal conductivity. The BTE approach successfully predicts the lattice thermal conductivity of bulk Si, whereas NEMD shows considerable underestimates. To evaluate the linear extrapolation method employed in NEMD to estimate bulk values, we analyze the size dependence in NEMD based on BTE calculations. We observe strong nonlinearity in the size dependence of NEMD in Si, which can be ascribed to acoustic phonons having long mean-free-paths and carrying considerable heat. Subsequently, we also apply the whole method to a thermoelectric material Mg2Si and demonstrate the reliability of the NEMD method for systems with low thermal conductivities.

Transcorrelated method for electronic systems coupled with variational Monte Carlo calculation

A Jastrow–Slater-type wave function is often used as a trial function for precise calculations of the total energy of electronic systems, where the correlation effect is taken into account by the Jastrow factor that directly depends on the distance between electrons. Since many-body integrals are inevitable there, the calculation totally depends on Monte Carlo sampling, and so, except for very simple cases, it is very difficult to optimize one-body wave functions in the Slater determinant which determine the nodal surfaces of the total wave function. Here we propose and demonstrate that the total wave function is efficiently optimized by coupling an ordinary variational Monte Carlo (VMC) technique with the transcorrelated method, in which the one-body wave functions are definitely obtained by solving Hartree–Fock-type self-consistent-field (SCF) equations derived from the similarity-transformed Hamiltonian. It is shown that the present method reproduces about 90% of the correlation energy for helium-like ...

Mechanism of electron attachment to van der Waals clusters: Application to carbon dioxide clusters

A theory on the attachment of very slow electrons to van der Waals clusters was developed on the basis of the electronic structure theory, and was applied to clarify the mechanism of the collisional electron transfer from a high‐Rydberg atom to a CO2 cluster. The strong coupled electron–phonon model is found to afford a reasonable mechanism of the attachment. The equilibrium geometry of (CO2)N (2≤N≤13) clusters are determined and their vertical affinity levels are obtained by the DV‐X α‐transition state method. Using this information, as well as some plausible assumptions on the values of the coupling constants, the attachment cross section σ is evaluated as a function of the energy of the incident electron. The theory predicts the existence of the threshold cluster size for the attachment and a sharp decrease of σ with the energy, which are consistent with the experimental results.

Two-electron reduction of a Rh-Mo-Rh dithiolato complex to form a triplet ground state associated with a change in CO coordination mode.

We synthesized a dithiolato-bridged heterometal trinuclear complex [{(eta(5)-C(5)Me(5))Rh(S(2)C(6)H(4))}(2)Mo(CO)(2)] (1) in which two rhodadithiolene complex units are bridged by a Mo(CO)(2) moiety. Complex 1 with a Rh(III)-Mo(0)-Rh(III) bond exhibits reversible one-step two-electron reduction with potential inversion. This redox process between 1 and 1(2-) accompanies a reversible structural change, which is an alternation in the CO coordination mode between semibridging and bridging. The ground state of dianion 1(2-) with a Rh(II)-Mo(0)-Rh(II) bond is assigned to spin triplet. These alternations of CO coordination mode and spin state are fully consistent with the density functional theory calculation results. This is the first example of multinuclear metalladithiolene complex which was successful in elucidating a reversible multielectron redox process associated with structural change and spin state change.

Reionization mechanism of neutralized He in low energy ion scattering spectroscopy

Abstract The reionization mechanism of neutralized He in low energy ion scattering spectroscopy is clarified on the basis of an ab initio CI potential energy calculation for some diatomic systems. It is newly reported that the antibonding interaction between the orbital and the core orbitals of the target atoms is important in the reionization mechanism.