Towards a systematic assessment of errors in diffusion Monte Carlo calculations of semiconductors: case study of zinc selenide and zinc oxide

TL;DR

This study systematically evaluates the errors in diffusion Monte Carlo calculations for semiconductors ZnSe and ZnO, focusing on fixed node errors, pseudopotential effects, and finite size effects, demonstrating controlled accuracy with modern computational methods.

Contribution

It provides a detailed assessment of various error sources in DMC calculations for semiconductors, establishing guidelines for accurate property predictions.

Findings

Errors can be controlled with modern computational resources.

Dirac-Fock pseudopotentials yield good estimates of cohesive energy and band gaps.

Differences in optical gaps depend on the pseudopotentials used.

Abstract

The fixed node diffusion Monte Carlo (DMC) method has attracted interest in recent years as a way to calculate properties of solid materials with high accuracy. However, the framework for the calculation of properties such as total energies, atomization energies, and excited state energies is not yet fully established. Several outstanding questions remain as to the effect of pseudopotentials, the magnitude of the fixed node error, and the size of supercell finite size effects. Here, we consider in detail the semiconductors ZnSe and ZnO and carry out systematic studies to assess the magnitude of the energy differences arising from controlled and uncontrolled approximations in DMC. The former include time step errors and supercell finite size effects for ground and optically excited states, and the latter include pseudopotentials, the pseudopotential localization approximation, and the fixed node approximation. We find that for these compounds, the errors can be controlled to good precision using modern computational resources, and that quantum Monte Carlo calculations using Dirac-Fock pseudopotentials can offer good estimates of both cohesive energy and the gap of these systems. We do however observe differences in calculated optical gaps that arise when different pseudopotentials are used.