Approaching Chemical Accuracy with Quantum Monte Carlo

TL;DR

This study demonstrates that quantum Monte Carlo methods, especially with optimized and multi-determinant trial wavefunctions, can achieve near chemical accuracy in predicting molecular atomization energies, surpassing previous computational approaches.

Contribution

The paper introduces improved quantum Monte Carlo techniques with optimized and multi-determinant trial wavefunctions to attain chemical accuracy in molecular energy calculations.

Findings

Mean absolute deviation reduced to 2.1 kcal/mol with orbital optimization.

Near chemical accuracy achieved with small active space wavefunctions (1.2 kcal/mol deviation).

Accuracy further improved using larger active spaces in phosphorus systems.

Abstract

A quantum Monte Carlo study of the atomization energies for the G2 set of molecules is presented. Basis size dependence of diffusion Monte Carlo atomization energies is studied with a single determinant Slater-Jastrow trial wavefunction formed from Hartree-Fock orbitals. With the largest basis set, the mean absolute deviation from experimental atomization energies for the G2 set is 3.0 kcal/mol. Optimizing the orbitals within variational Monte Carlo improves the agreement between diffusion Monte Carlo and experiment, reducing the mean absolute deviation to 2.1 kcal/mol. Moving beyond a single determinant Slater-Jastrow trial wavefunction, diffusion Monte Carlo with a small complete active space Slater-Jastrow trial wavefunction results in near chemical accuracy. In this case, the mean absolute deviation from experimental atomization energies is 1.2 kcal/mol. It is shown from calculations on systems containing phosphorus that the accuracy can be further improved by employing a larger active space.