Beyond chemical accuracy: The pseudopotential approximation in diffusion Monte Carlo calculations of the HCP to BCC phase transition in beryllium

TL;DR

This paper investigates the impact of pseudopotential approximations in diffusion Monte Carlo calculations of beryllium's phase transition, aiming to improve accuracy and address discrepancies with experimental data.

Contribution

It identifies the pseudopotential approximation as the main source of error and demonstrates that removing it yields results aligning closely with experiments.

Findings

Excellent agreement with experimental data for the HCP phase.

Results comparable to density functional theory calculations for phase transition pressure.

Highlighting the importance of pseudopotential treatment in diffusion Monte Carlo methods.

Abstract

Motivated by the disagreement between recent diffusion Monte Carlo calculations and experiments on the phase transition pressure between the ambient and beta-Sn phases of silicon, we present a study of the HCP to BCC phase transition in beryllium. This lighter element provides an oppor- tunity for directly testing many of the approximations required for calculations on silicon and may suggest a path towards increasing the practical accuracy of diffusion Monte Carlo calculations of solids in general. We demonstrate that the single largest approximation in these calculations is the pseudopotential approximation. After removing this we find excellent agreement with experiment for the ambient HCP phase and results similar to careful calculations using density functional theory for the phase transition pressure.