  The Dougherty model Fokker-Planck operator is extended to describe nonlinear
full-f collisions between multiple species in plasmas. Simple relations for
cross-species interactions are developed which obey conservation laws, and
reproduce familiar velocity and temperature relaxation rates. This treatment of
multispecies Dougherty collisions, valid for arbitrary mass ratios, satisfies
the H-Theorem unlike analogous Bhatnagar-Gross-Krook operators.
