  Accurate calculation of the ion-ion recombination rate coefficient has been
of long-standing interest, as it controls the ion concentration in gas phase
systems and in aerosols. We describe the development of a hybrid
continuum-molecular dynamics approach to determine the ion-ion recombination
rate coefficient. The approach is based on the limiting sphere method
classically used for transition regime collision phenomena in aerosols. When
ions are sufficiently far from one another, ion-ion relative motion is
described by diffusion equations while within a critical distance, molecular
dynamics (MD) simulations are used to model ion-ion motion. MD simulations are
parameterized using the AMBER force-field as well as by considering partial
charges on atoms. Ion-neutral gas collisions are modeled in two mutually
exclusive cubic domains composed of 103 gas atoms each, which remain centered
on the recombining ions throughout calculations. Example calculations are
reported for NH4+ recombination with NO2- in He, across a pressure range from
10 kPa to 10,000 kPa. Excellent agreement is found in comparison of
calculations to literature values for the 100 kPa recombination rate
coefficient (1.0 x 10-12 m3 s-1) in He. We also recover the experimentally
observed increase in recombination rate coefficient with pressure at
sub-atmospheric pressures, and the observed decrease in recombination rate
coefficient in the high pressure continuum limit. We additionally find that
non-dimensionalized forms of rate coefficients are consistent with recently
developed equations for the dimensionless charged particle-ion collision rate
coefficient based on Langevin dynamics simulations.
