Full- and reduced-dimensionality instanton calculations of the tunnelling splitting in the formic acid dimer

TL;DR

This paper uses the ring-polymer instanton method to accurately compute tunnelling splittings in formic acid dimers, demonstrating the importance of including multiple vibrational modes for precise results.

Contribution

It applies full- and reduced-dimensionality instanton calculations to the formic acid dimer, showing the necessity of multiple modes for accurate tunnelling splitting predictions.

Findings

Full-dimensional instanton results match experimental data within 20%.

Nearly all vibrational modes influence the tunnelling splitting.

Reduced-dimensionality approaches converge as more degrees of freedom are included.

Abstract

The ring-polymer instanton approach is applied to compute the ground-state tunnelling splitting of four isotopomers of the formic acid dimer using the accurate PES of Qu and Bowman [Phys. Chem. Chem. Phys., 2016, 18, 24835]. As well as performing the calculations in full dimensionality, we apply a reduced-dimensionality approach to study how the results converge as successively more degrees of freedom are included. The instanton approximation compares well to exact quantum results where they are available but shows that nearly all the modes are required to quantitatively obtain the tunnelling splitting. The full-dimensional instanton calculation reproduces the experimental results, with an error of only about 20 percent.