Computing Hydrogen Tunneling Splittings with Nuclear-Electronic Orbital Multireference Configuration Interaction

TL;DR

This paper introduces the NEO-MRCI computational method to accurately calculate hydrogen tunneling splittings by treating nuclei and electrons quantum mechanically, validated against exact calculations for various systems.

Contribution

The paper develops and applies the NEO-MRCI method for computing vibronic energies in hydrogen tunneling systems, demonstrating its accuracy and potential for studying tunneling phenomena.

Findings

NEO-MRCI accurately reproduces grid-based tunneling splittings.

Method effectively captures static and dynamic electron-proton correlations.

Applicable to multiple hydrogen tunneling systems.

Abstract

Hydrogen tunneling is an important process that impacts reaction rates and molecular spectra. Describing and understanding this process requires a quantum mechanical treatment of the transferring hydrogen. The nuclear-electronic orbital (NEO) approach treats specified nuclei quantum mechanically on the same level as electrons and has recently been implemented at the multireference configuration interaction (MRCI) wavefunction level. The NEO-MRCI method includes both the static correlation necessary to describe hydrogen tunneling and the electron-proton dynamic correlation required for computing quantitatively accurate nuclear-electronic vibronic states. Herein, the NEO-MRCI method is used to compute the nuclear-electronic wavefunctions and corresponding vibronic energies for four hydrogen tunneling systems at fixed geometries for a range of donor-acceptor distances. Comparison of the NEO-MRCI results to numerically exact grid-based calculations shows that the NEO-MRCI method can be used to obtain accurate hydrogen and deuterium tunneling splittings at fixed geometries. Thus, this work presents an important component for studying hydrogen tunneling systems.