  A model for the description of proton collisions from molecules composed of
atoms such as hydrogen, carbon, nitrogen, oxygen and phosphorus (H, C, N, O, P)
was recently extended to treat collisions with multiply charged ions with a
focus on net ionization. Here we complement the work by focusing on net
capture. The ion-atom collisions are computed using the two-center basis
generator method. The atomic net capture cross sections are then used to
assemble two models for ion-molecule collisions: an independent atom model
(IAM) based on the Bragg additivity rule (labeled IAM-AR), and also the
so-called pixel-counting method (IAM-PCM) which introduces dependence on the
orientation of the molecule during impact. The IAM-PCM leads to significantly
reduced capture cross sections relative to IAM-AR at low energies, since it
takes into account the overlap of effective atomic cross sectional areas. We
compare our results with available experimental and other theoretical data
focusing on water vapor (H2O), methane (CH4) and uracil (C4H4N2O2). For the
water molecule target we also provide results from a classical-trajectory Monte
Carlo approach that includes dynamical screening effects on projectile and
target. For small molecules dominated by a many-electron atom, such as carbon
in methane, or oxygen in water we find a saturation phenomenon for higher
projectile charges (Q = 3) and low energies, where the net capture cross
section for the molecule is dominated by the net cross section for the
many-electron atom, and the net capture cross section is not proportional to
the total number of valence electrons.
