# Accurate rate coefficients for models of interstellar gas-grain chemistry

## TL;DR

This paper presents accurate, easy-to-use expressions for reaction rate coefficients in interstellar gas-grain chemistry models, improving upon previous approximations and aligning well with detailed simulations.

## Contribution

It introduces simplified, precise rate coefficient formulas derived from exact solutions, enhancing modeling accuracy for interstellar chemistry.

## Key findings

- New rate coefficients match kinetic Monte Carlo simulations
- Formulas account for grain size and fluctuation effects
- Applicable to reactions involving multiple species

## Abstract

The methodology for modeling grain-surface chemistry has been greatly improved by taking into account the grain size and fluctuation effects. However, the reaction rate coefficients currently used in all practical models of gas-grain chemistry are inaccurate by a significant amount. We provide expressions for these crucial rate coefficients that are both accurate and easy to incorporate into gas-grain models. We use exact results obtained in earlier work, where the reaction rate coefficient was defined by a first-passage problem, which was solved using random walk theory. The approximate reaction rate coefficient presented here is easy to include in all models of interstellar gas-grain chemistry. In contrast to the commonly used expression, the results that it provides are in perfect agreement with detailed kinetic Monte Carlo simulations. We also show the rate coefficient for reactions involving multiple species.