Speeding up ab initio diffusion Monte Carlo simulations by a smart lattice regularization

One of the most significant drawbacks of the all-electron ab initio diffusion Monte Carlo (DMC) is that its computational cost drastically increases with the atomic number (Z), which typically scales with Z(similar to 6). In this study, we introduce a very efficient implementation of the lattice regularized diffusion Monte Carlo (LRDMC), where the conventional time discretization is replaced by its lattice space counterpart. This scheme enables us to conveniently adopt a small lattice space in the vicinity of nuclei, and a large one in the valence region, by which a considerable speedup is achieved, especially for large atomic number Z. Indeed, the computational performances of the improved LRDMC can be theoretically established based on the Thomas-Fermi model for heavy atoms, implying the optimal Z(similar to 5) scaling for all-electron DMC calculations. This improvement enables us to apply the DMC technique even for superheavy elements (Z >= 104), such as oganesson (Z = 118), which has the highest atomic number of all synthesized elements so far

Interface modelling for {\it ab initio} evaluation of contact angle on a metallic surface

Controlling the contact angles of the wettability is an important issue especially in industrial applications. Establishing its {\it ab initio} predictions is hence a topic of great interest. For the predictions, it is required to setup a model of the adsorption structure of liquid molecules on a surface. The appropriate setting is expected to depend on whether the surface is of insulating or metallic materials, the latter of which is the target of the present study while all preceding {\it ab initio} studies have worked on the former. Since the feasibility of {\it ab initio} evaluations relies on the approximation of the liquid-gas interface energy evaluated roughly by the crystal ice, it would be a natural choice to take the periodic honeycomb array of the water molecules as the adsorbing model of water on the surface. Although the periodic model have successfully been used for the preceding treatments of insulating surfaces, we found for the case with metallic surfaces that the periodic model gives worse prediction to reproduce experimental values. Rather than that, the models with isolated water multimers are found to give better predictions. The ambiguity of the models about the size of multimers and the coverage is found to be small ($\sim\pm 10^{\circ}$), and is averaged over to give a plausible value based on the Boltzmann weight with the adsorbing energies. The procedure we are providing can generally be applicable to any of wettability on the surfaces of metallic materials.Comment: 4 page

Atomic forces by quantum Monte Carlo: application to phonon dispersion calculation

We report the first successful application of the {\it ab initio} quantum Monte Carlo (QMC) framework to a phonon dispersion calculation. A full phonon dispersion of diamond is successfully calculated at the variational Monte Carlo (VMC) level, based on the frozen-phonon technique. The VMC-phonon dispersion is in good agreement with the experimental results, giving renormalized harmonic optical frequencies very close to the experimental values, by significantly improving upon density functional theory (DFT) in the generalized gradient approximation. Key to success for the QMC approach is the statistical error reduction in atomic force evaluation. We show that this can be achieved by using well conditioned atomic basis sets, by explicitly removing the basis-set redundancy, which reduces the statistical error of forces by up to two orders of magnitude. This leads to affordable and accurate QMC-phonons calculations, up to $10^{4}$ times more efficient than previous attempts, and paves the way to new applications, particularly in correlated materials, where phonons have been poorly reproduced so far.Comment: 10 page

Towards chemical accuracy using the Jastrow correlated antisymmetrized geminal power ansatz

Herein, we report accurate atomization energy calculations for 55 molecules in the Gaussian-2 (G2) set using lattice regularized diffusion Monte Carlo (LRDMC). We compare the Jastrow-Slater determinant ansatz with a more flexible JsAGPs (Jastrow correlated antisymmetrized geminal power with singlet correlation) ansatz. AGPs is built from pairing functions, which explicitly include pairwise correlations among electrons and hence, this ansatz is expected to be more efficient in recovering the correlation energy. The AGPs wave functions are first optimized at the variational Monte Carlo (VMC) level, which includes both the Jastrow factor and the nodal surface optimization. This is followed by the LRDMC projection of the ansatz. Remarkably, for many molecules, the LRDMC atomization energies obtained using the JsAGPs ansatz reach chemical accuracy ($\sim$1 kcal/mol) and for most other molecules, the atomization energies are accurate within $\sim$5 kcal/mol. We obtained a mean absolute deviation of 1.6 kcal/mol with JsAGPs and 3.2 kcal/mol with JDFT (Jastrow factor + Slater determinant with DFT orbitals) ansatz. This work shows the effectiveness of the flexible AGPs ansatz for atomization energy calculations and electronic structure simulations in general.Comment: 15 pages, 4 figures, JCTC accepted version after peer-revie

Potential high-TcT_{c} superconductivity in YCeH20_{20} and LaCeH20_{20} under pressure

Lanthanum, yttrium, and cerium hydrides are the three most well-known superconducting binary hydrides, which have gained great attention in both theoretical and experimental studies. Recent studies have shown that ternary hydrides composed of lanthanum and yttrium can achieve high superconductivity around 253 K. In this study, we employ the evolutionary-algorithm-based crystal structure prediction (CSP) method and first-principles calculations to investigate the stability and superconductivity of ternary hydrides composed of (Y, Ce) and (La, Ce) under high pressure. Our calculations show that there are multiple stable phases in Y-Ce-H and La-Ce-H hydrides, among which $P4/mmm$-YCeH$_{8}$, $P4/mmm$-LaCeH$_{8}$, $R\bar{3}m$-YCeH$_{20}$, and $R\bar{3}m$-LaCeH$_{20}$ possessing H$_{18}$ or H$_{32}$ clathrate structures can maintain both of the thermodynamic and dynamic stabilities. In addition, we also find that these phases also maintain a strong resistance to decomposition at high temperature. Electron-phonon coupling calculations show that all of these four phases can exhibit high-temperature superconductivity. $R\bar{3}m$-YCeH$_{20}$ is predicted to have a superconducting transition temperature ($T_{c}$) as high as 246 K at 350 GPa. The $T_{c}$ value of $R\bar{3}m$-LaCeH$_{20}$ at 250 GPa is about 233 K, which is slightly smaller than that of $R\bar{3}m$-YCeH$_{20}$. However, it is found that $R\bar{3}m$-LaCeH$_{20}$ can be stabilized at 200 GPa, making the high-pressure synthesis of LaCeH$_{20}$ easier.Comment: 5 figure

二酸化炭素の利用に向けたオレフィン重合反応に関する研究

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 野崎 京子, 東京大学教授 相田 卓三, 東京大学教授 西林 仁昭, 東京大学准教授 吉尾 正史, 東京工業大学准教授 竹内 大介, Ghent University教授 Steven P.NolanUniversity of Tokyo(東京大学