Pressure-induced diamond to beta-tin transition in bulk silicon: a near-exact quantum Monte Carlo study

TL;DR

This study uses a near-exact quantum Monte Carlo method to accurately determine the pressure-induced phase transition from diamond to beta-tin in silicon, achieving results that align well with experimental data.

Contribution

It demonstrates the application of the phaseless auxiliary-field quantum Monte Carlo method to accurately compute phase transition pressures in silicon, reducing fixed-node errors present in previous methods.

Findings

Transition pressure agrees with experimental measurements

Systematic error in total energies is within 0.5 mHa/atom

Method effectively benchmarks and controls quantum Monte Carlo errors

Abstract

The pressure-induced structural phase transition from diamond to beta-tin in silicon is an excellent test for theoretical total energy methods. The transition pressure provides a sensitive measure of small relative energy changes between the two phases (one a semiconductor and the other a semimetal). Experimentally, the transition pressure is well characterized. Density-functional results have been unsatisfactory. Even the generally much more accurate diffusion Monte Carlo method has shown a noticeable fixed-node error. We use the recently developed phaseless auxiliary-field quantum Monte Carlo (AFQMC) method to calculate the relative energy differences in the two phases. In this method, all but the error due to the phaseless constraint can be controlled systematically and driven to zero. In both structural phases we were able to benchmark the error of the phaseless constraint by carrying out exact unconstrained AFQMC calculations for small supercells. Comparison between the two shows that the systematic error in the absolute total energies due to the phaseless constraint is well within 0.5 mHa/atom. Consistent with these internal benchmarks, the transition pressure obtained by the phaseless AFQMC from large supercells is in very good agreement with experiment.