Assessing the accuracy of compound formation energies with quantum Monte Carlo

TL;DR

This study evaluates the accuracy of quantum Monte Carlo methods in predicting compound formation energies, comparing results with density functional theory and experiments for intermetallic VPt2 and semiconductor CuI.

Contribution

It demonstrates the application of many-body quantum Monte Carlo with single-reference trial functions to compute formation energies of complex compounds, highlighting its advantages over standard DFT methods.

Findings

QMC agrees with some DFT estimates but not with experimental values for VPt2.

QMC results for CuI differ from DFT, indicating exchange-correlation biases.

Incorporating spin-orbit corrections improves agreement with experimental data.

Abstract

Accurately predicting the formation energy of a compound, which describes its thermodynamic stability, is a key challenge in materials physics. Here, we employ many-body quantum Monte Carlo (QMC) with single-reference trial functions to compute the formation energy of two electronically disparate compounds, the intermetallic VPt$_2$ and the semiconductor CuI, for which standard density functional theory (DFT) predictions using both the Perdew-Burke Ernzerhof (PBE) and the strongly constrained and appropriately normed (SCAN) density functional approximations deviate markedly from available experimental values. For VPt$_2$, we find an agreement between QMC, SCAN, and PBE0 estimates, which therefore remain in disagreement with the much less exothermic experimental value. For CuI, the QMC result agrees with neither SCAN nor PBE pointing towards DFT exchange-correlation biases, likely related to the localized Cu $3d$ electrons. Compared to the behavior of some density functional approximations within DFT, spin-averaged QMC exhibits a smaller but still appreciable deviation when compared to experiment. The QMC result is slightly improved by incorporating spin-orbit corrections for CuI and solid I$_2$, so that experiment and theory are brought into imperfect but reasonable agreement within about 120~meV/atom.