# A simple local expression for the prefactor in transition state theory

## TL;DR

This paper introduces a computationally efficient method to calculate the frequency prefactor in transition state theory by focusing on a localized active region and applying a correction, reducing computational costs.

## Contribution

The authors develop a simple partial phonon DOS approach for the prefactor calculation, avoiding full phonon calculations and enabling easier integration into existing simulation tools.

## Key findings

- The method accurately predicts the prefactor with reduced computational effort.
- Convergence improves with larger active regions, balancing accuracy and cost.
- Open source implementation available for LAMMPS.

## Abstract

We present a simple and accurate computational technique to determine the frequency prefactor in harmonic transition state theory without necessitating full phonon density of states (DOS) calculations. The atoms in the system are partitioned into an "active region", where the kinetic process takes place, and an "environment" surrounding the active region. It is shown that the prefactor can be obtained by a partial phonon DOS calculation of the active region with a simple correction term accounting for the environment, under reasonable assumptions regarding atomic interactions. Convergence with respect to the size of the active region is investigated for different systems, as well as the reduction in computational costs when compared to full phonon DOS calculation. Additionally, we provide an open source implementation of the algorithm that can be added as an extension to LAMMPS software.