  The mutual neutralization of H^+ and H^- ions at low collision energies is
studied by means of a molecular close-coupling approach. All degrees of freedom
are treated at the full quantum level taking into account also the identity of
the nuclei. The relevant ^1Sigma_g,u^+ electronic states and their associated
nonadiabatic radial couplings are calculated for internuclear distances between
0.5 and 50 a_0. Following a transformation into a strictly diabatic basis,
these quantities enter into a set of coupled equations for the motion of the
nuclei. Numerical solution of these equations allows the cross sections for
scattering into the H(1)+H^*(n), n=1,2,3 channels to be calculated. In the
present paper, results are reported for the collision energy region 0.001 to
100 eV, with special emphasis on the important energy region below 10 eV. The
low temperature rate coefficient is obtained from a parametrization of the
calculated neutralization cross section and is estimated to be valid in the
range 10 to 10 000 K.
