arXiv subjects

Hideaki Takahashi

Publications and source records attributed to Hideaki Takahashi.

21 records · Page 2Linked to original sources

arXiv2020

Open Quantum Dynamics Theory of Spin Relaxation: Application to $μ$SR and Low-Field NMR Spectroscopies

An open quantum system refers to a system, which is in turn coupled to an environment that can describe time irreversible dynamics through which the system evolves toward the thermal equilibrium state. We present a quantum mechanically rigorous theory in order to help an analysis of spectra obtained from the advanced nuclear magnetic resonance (NMR) and muon spin rotation, relaxation or resonance ($μ$SR) techniques. Our approach is based on the numerically "exact" hierarchical equations of motion (HEOM) approach, which allows us to study the reduced system dynamics for non-perturbative and non-Markovian system-bath interactions at finite temperature even under strong time-dependent perturbations. We demonstrate the present theory to analyze $μ$SR and low-field NMR spectra, as an extension of the Kubo-Toyabe theory focusing on the effects of temperature and anisotropy of a local magnetic field, to help further the development of these experimental means.

cond-mat.stat-mech↗

arXiv2018

A Simple and Effective Solution to the Constrained QM/MM Simulations

It is a promising extension of the quantum mechanical/molecular mechanical (QM/MM) approach to incorporate the solvent molecules surrounding the QM solute into the QM region to ensure the adequate description of the electronic polarization of the solute. However, the solvent molecules in the QM region inevitably diffuse into the MM bulk during the QM/MM simulation. In this article we developed a simple and efficient method, referred to as boundary constraint with correction (BCC), to prevent the diffusion of the solvent water molecules by means of a constraint po- tential. The point of the BCC method is to compensate the error in a statistical property due to the bias potential by adding a correction term obtained through a set of QM/MM simulations. The BCC method is designed so that the effect of the bias potential completely vanishes when the QM solvent is identical with the MM solvent. Furthermore, the desirable conditions, that is, the continuities of energy and force and the conservations of energy and momentum, are fulfilled in principle. We applied the QM/MM-BCC method to a hydronium ion in aqueous solution to construct the radial distribution function(RDF) of the solvent around the solute. It was demonstrated that the correction term fairly compensated the error and led the RDF in good agreement with the result given by an ab initio molecular dynamics simulation.

cond-mat.soft↗

arXiv2016

Density Functional Theory Based on the Electron Distribution on the Energy Coordinate

We introduced a new electron density n(ε) by projecting the spatial electron density n(r) onto the energy coordinate ε defined with the external potential υ(r) of interest. Then, a density functional theory (DFT) was formulated, where n(ε) serves as a fundamental variable for the electronic energy. It was demonstrated that the Kohn-Sham equation can also be adapted to the DFT that employs the density n(ε) as an argument to the exchange energy functional. An important attribute of the energy density is that it involves the spatially non-local population of the spin-adapted density n(r) at the bond dissociation. By taking advantage of this property we developed a prototype of the static correlation functional employing no empirical parameters, which realized a reasonable dissociation curve for H2 molecule.

physics.chem-ph↗