Accurate barrier heights using diffusion Monte Carlo

TL;DR

This study demonstrates that diffusion Monte Carlo can accurately compute barrier heights with minimal nodal error, showing robustness across different mean-field wave functions and identifying the HOMO-LUMO gap as a key error descriptor.

Contribution

It provides a systematic assessment of fixed node DMC for barrier heights and identifies the HOMO-LUMO gap as a predictor of nodal error, improving understanding of DMC accuracy.

Findings

DMC yields about 1.5 kcal/mol error in barrier heights.

Results are robust to different mean-field wave functions.

HOMO-LUMO gap correlates with nodal error.

Abstract

Fixed node diffusion Monte Carlo (DMC) has been performed on a test set of forward and reverse barrier heights for 19 non-hydrogen-transfer reactions, and the nodal error has been assessed. The DMC results are robust to changes in the nodal surface, as assessed by using different mean-field techniques to generate single determinant wave functions. Using these single determinant nodal surfaces, DMC results in errors of 1.5(5) kcal/mol on barrier heights. Using the large data set of DMC energies, we attempted to find good descriptors of the fixed node error. It does not correlate with a number of descriptors including change in density, but does correlate with the gap between the highest occupied and lowest unoccupied orbital energies in the mean-field calculation.