How Large are Nonadiabatic Effects in Atomic and Diatomic Systems?

TL;DR

This paper uses advanced quantum Monte Carlo methods to quantify nonadiabatic effects on the energies of atomic and molecular systems, revealing small but measurable deviations from the Born-Oppenheimer approximation.

Contribution

It provides the first high-accuracy nonadiabatic energy calculations for a range of atomic and diatomic systems using fixed-node diffusion Monte Carlo.

Findings

Ionization energies are nearly unaffected by nonadiabatic effects.

Atomization energies show small nonadiabatic contributions.

Results demonstrate the accuracy of nonadiabatic quantum Monte Carlo methods.

Abstract

With recent developments in simulating nonadiabatic systems to high accuracy, it has become possible to determine how much energy is attributed to nuclear quantum effects beyond zero-point energy. In this work we calculate the non-relativistic ground-state energies of atomic and molecular systems without the Born-Oppenheimer approximation. For this purpose we utilize the fixed-node diffusion Monte Carlo method, in which the nodes depend on both the electronic and ionic positions. We report ground-state energies for all systems studied, ionization energies for the first-row atoms and atomization energies for the first-row hydrides. We find the ionization energies of the atoms to be nearly independent of the Born-Oppenheimer approximation, within the accuracy of our results. The atomization energies of molecular systems, however, show small effects of the nonadiabatic coupling between electrons and nuclei.