Semi-stochastic full configuration interaction quantum Monte Carlo: developments and application

TL;DR

This paper advances semi-stochastic FCIQMC methods by developing a new deterministic space generation approach, improving efficiency and parallel performance, and applying it to compute accurate energies for molecular systems.

Contribution

It introduces an alternative deterministic space generation method without prior wave function knowledge and demonstrates efficiency gains in stochastic sampling and energy calculations.

Findings

Enhanced efficiency in reduced density matrix calculations.

Improved parallel performance in semi-stochastic FCIQMC.

Achieved near-wavenumber accuracy in Beryllium dimer energies.

Abstract

We expand upon the recent semi-stochastic adaptation to full configuration interaction quantum Monte Carlo (FCIQMC). We present an alternate method for generating the deterministic space without a priori knowledge of the wave function and present stochastic efficiencies for a variety of both molecular and lattice systems. The algorithmic details of an efficient semi-stochastic implementation are presented, with particular consideration given to the effect that the adaptation has on parallel performance in FCIQMC. We further demonstrate the benefit for calculation of reduced density matrices in FCIQMC through replica sampling, where the semi-stochastic adaptation seems to have even larger efficiency gains. We then combine these ideas to produce explicitly correlated corrected FCIQMC energies for the Beryllium dimer, for which stochastic errors on the order of wavenumber accuracy are achievable.