  A mean-field model to describe electron transfer processes in ion-molecule
collisions at the $\hbar =0$ level is presented and applied to collisions
involving water and ammonia molecules. Multicenter model potentials account for
the molecular structure and geometry. They include charge screening parameters
which in the most advanced version of the model depend on the instantaneous
degree of ionization so that dynamical screening effects are taken into
account. The work is implemented using the classical-trajectory Monte Carlo
method, i.e., Hamilton's equations are solved for classical statistical
ensembles that represent the initially populated orbitals. The time-evolved
trajectories are sorted into ionizing and electron capture events, and a
multinomial analysis of the ensuing single-particle probabilities is employed
to calculate differential and total cross sections for processes that involve
single- and multiple-electron transitions. Comparison is made with experimental
data and some previously reported calculations to shed light on the
capabilities and limitations of the approach.
