First order approximation of polyatomic ion and neutral diffusion in neutral gases under arbitrary fields

Abstract

This work presents a trajectory-based method for predicting diffusion coefficients of ions and neutral molecules in dilute gases from the thermal limit to strongly driven conditions. Classical trajectory simulations are coupled with two-temperature kinetic theory to evaluate the collision integrals required by the generalized Einstein relations. Polyatomic ion geometries and charge distributions obtained from density functional theory are incorporated directly into 4–6–12 interaction potentials, allowing a consistent treatment of molecular structure, long-range interactions, and energy transfer during collisions. The method is implemented in the Mass Diffusivity Software (MaDiS) and validated against experimental data for both neutral and ionic systems across a wide range of conditions. For neutral molecules in the zero-field limit, the calculated diffusion coefficients reproduce experimental trends across multiple gases and temperatures with an average deviation of ∼5%, despite the use of non-optimized Lennard-Jones parameters. The simulations correctly capture the temperature dependence of diffusivity and remain consistent across a wide range of molecular sizes. Under applied electric fields, the method captures the departure from equilibrium and the resulting anisotropic transport, providing direct predictions of longitudinal and transverse diffusion coefficients. For monoatomic ions, longitudinal diffusion coefficients are reproduced with good agreement over substantial ranges of reduced electric field, typically up to 300–400 Td depending on the ion–gas system, consistent with benchmark datasets. Comparable behavior is obtained for polyatomic ions, demonstrating that the combination of trajectory-derived collision dynamics and the two-temperature formalism remains applicable when realistic molecular structure is included.