  A theoretical method for the estimation of cross sections and rates for
excitation and charge transfer processes in low-energy hydrogen atom collisions
with neutral atoms, based on an asymptotic two-electron model of ionic-covalent
interactions in the neutral atom-hydrogen atom system, is presented. The
calculation of potentials and non-adiabatic radial couplings using the method
is demonstrated. The potentials are used together with the multi-channel
Landau-Zener model to calculate cross sections and rate coefficients. The main
feature of the method is that it employs asymptotically exact atomic
wavefunctions, which can be determined from known atomic parameters. The method
is applied to Li+H, Na+H, and Mg+H collisions, and the results compare well
with existing detailed full-quantum calculations. The method is applied to the
astrophysically important problem of Ca+H collisions, and rate coefficients are
calculated for temperatures in the range 1000-20000 K.
