Modeling approaches for atmospheric ion–dipole collisions: all-atom trajectory simulations and central field methods

Abstract

Abstract. Ion–dipole collisions can facilitate the formation of atmospheric aerosol particles and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models, or simple parametrizations, have been used to calculate the rate coefficients of ion–dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners. To determine the accuracy and applicability of these approaches under atmospheric conditions, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectories, sampling the relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles and charged and neutral dipolar clusters. Based on the good agreement between collision cross sections and rate coefficients obtained from molecular dynamics trajectories and a generalized central field model, we conclude that the effective interactions between the collision partners are isotropic to a high degree, and the model is able to capture the relevant physicochemical properties of the systems. In addition, when the potential of mean force is recalculated at the respective temperatures, the central field model exhibits the correct temperature dependence of the collision process. The classical parametrization by Su and Chesnavich (1982), which combines a central field model with simplified trajectory simulations, is able to predict the collision rate coefficients and their temperature dependence quite well for molecular systems, but the agreement worsens for systems containing clusters. Based on our results, we propose the combination of potential of mean force calculation and a central field model as a viable and elegant alternative to the brute force sampling of individual collision trajectories over a large range of impact parameters and relative velocities.

Collision-sticking rates of acid–base clusters in the gas phase determined from atomistic simulation and a novel analytical interacting hard-sphere model

“…We have previously shown that the central field approach yields very similar results to those from sampling collision trajectories using MD, provided that the attractive interaction used in the central field model is fitted to the tail of the potential of mean force (PMF) between the collision partners (Neefjes et al, 2022). The largest collision partners studied in that study were, however, dimers.…”

“…Molecular dynamics (MD) simulations allow us to study the time evolution of collision systems, where all intra-and intermolecular interactions are described through force fields. Recently, we have developed several atomistic molecular dynamics simulation frameworks (Yang et al, 2018;Halonen et al, 2019;Neefjes et al, 2022) to determine collision probabilities from sampling trajectories of atoms or molecules, which predict enhanced collision rate coefficients in good agreement with experiments (Lehtipalo et al, 2016;Stolzenburg et al, 2020). However, as the adequate sampling of collision probabilities requires simulating a large number of binary collisions, these approaches are computationally expensive.…”

“…To calculate collision and sticking probabilities in atomistic molecular dynamics simulations we need simple, but robust, criteria for when two collision partners have collided and when they are sticking together, as the large number of individual trajectories makes a manual inspection unfeasible. Collision/sticking criteria can be based on geometric and/or energetic properties of the system (Yang et al, 2018;Halonen et al, 2019;Neefjes et al, 2022). Here, we considered a criterion based on the center of mass distance between the collision partners, as this does not require knowledge of the detailed bonding between monomers and clusters of different sizes and can be directly related to the analytical interacting hardsphere model.…”

Ion–Molecule Rate Constants for Reactions of Sulfuric Acid with Acetate and Nitrate Ions

“… 67 This additional attractive potential between the ion and the neutral molecule increased the collision cross-sectional area and alter the angle of collision between the ion and neutral molecule. Furthermore, preliminary molecular dynamic simulations following Neefjes et al 68 suggest a higher collision rate constant for acetate ions (with and without water ligand) with sulfuric acid compared to nitrate dimer ion due in part to acetate’s dipole moment. A forthcoming molecular dynamic simulation study will examine the role ion’s dipole and composition play on collision rate constants with molecular dipoles.…”

Further cautionary tales on thermostatting in Molecular Dynamics: energy equipartitioning and non-equilibrium processes in gas-phase simulations

“…In this study, we explore issues related to both obtaining and maintaining thermal equilibrium when performing MD simulations of bimolecular reactions in the gas phase, close to the free molecular regime. 32,33 We focus on simulations of flexible molecular compounds with explicit internal dof, i.e., vibrational modes. First, to study issues related to establishing thermal equilibrium and equipartitioning, we tested the often-used Nosé-Hoover, Canonical Sampling through Velocity Rescaling, and Langevin thermostats, using a system of 30 As the aim of this study is primarily methodological, the potential problems related to thermostatted simulations are investigated using a computationally inexpensive classical force field approach, rather than a chemically more realistic reactive force field or a quantum mechanical description of the compounds and their interactions.…”