Non-adiabatic effects within a single thermally-averaged potential energy surface: Thermal expansion and reaction rates of small molecules

TL;DR

This paper introduces a temperature-dependent effective potential that accounts for multiple electronic states, revealing significant impacts on molecular properties like bond length and reaction rates at finite temperatures.

Contribution

It develops a simple, accurate method to include low-lying electronic states in thermodynamic calculations, improving predictions of molecular behavior beyond the ground-state approximation.

Findings

Thermal expansion of Mn₂ is significantly affected by excited states.

Reaction rates of ozone are modified by up to 20% when including excited electronic states.

The method accurately reproduces electronic state populations at finite temperatures.

Abstract

At non-zero temperature and when a system has low-lying excited electronic states, the ground-state Born--Oppenheimer approximation breaks down and the low-lying electronic states are involved in any chemical process. In this work, we use a temperature-dependent effective potential for the nuclei which can accomodate the influence of an arbitrary number of electronic states in a simple way, while at the same time producing the correct Boltzmann equibrium distribution for the electronic part. With the help of this effective potential, we show that thermally-activated low-lying electronic states can have a significant effect in molecular properties for which electronic excitations are oftentimes ignored. We study the thermal expansion of the Manganese dimer, Mn$_2$, where we find that the average bond length experiences a change larger than the present experimental accuracy upon the inclusion of the excited states into the picture. We also show that, when these states are taken into account, reaction rate constants are modified. In particular, we study the opening of the ozone molecule, O$_3$, and show that in this case the rate is modified as much as a 20% with respect to the ground-state Born--Oppenheimer prediction.