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Accurate calculation of the ion-ion recombination rate coefficient has beenof long-standing interest, as it controls the ion concentration in gas phasesystems and in aerosols. We describe the development of a hybridcontinuum-molecular dynamics approach to determine the ion-ion recombinationrate coefficient. The approach is based on the limiting sphere methodclassically used for transition regime collision phenomena in aerosols. Whenions are sufficiently far from one another, ion-ion relative motion isdescribed by diffusion equations while within a critical distance, moleculardynamics (MD) simulations are used to model ion-ion motion. MD simulations areparameterized using the AMBER force-field as well as by considering partialcharges on atoms. Ion-neutral gas collisions are modeled in two mutuallyexclusive cubic domains composed of 103 gas atoms each, which remain centeredon the recombining ions throughout calculations. Example calculations arereported for NH4+ recombination with NO2- in He, across a pressure range from10 kPa to 10,000 kPa. Excellent agreement is found in comparison ofcalculations to literature values for the 100 kPa recombination ratecoefficient (1.0 x 10-12 m3 s-1) in He. We also recover the experimentallyobserved increase in recombination rate coefficient with pressure atsub-atmospheric pressures, and the observed decrease in recombination ratecoefficient in the high pressure continuum limit. We additionally find thatnon-dimensionalized forms of rate coefficients are consistent with recentlydeveloped equations for the dimensionless charged particle-ion collision ratecoefficient based on Langevin dynamics simulations.

Calculation of the Ion-Ion Recombination Rate Coefficient via a Hybrid Continuum- Molecular Dynamics Approach