Quasi-adiabatic basis functions for the S-matrix Kohn variational approach to quantum reactive scattering

A basis set contraction scheme is described for carrying out reactive scattering calculations within the S-matrix version of the Kohn variational principle. The new basis functions are distributed Gaussians for the radial (translational) degrees of freedom, as used before, but with quasi-adiabatic internal eigenfunctions for the internal degrees of freedom; i.e., each translational Gaussian has a different internal function, the adiabatic internal eigenfunction for that translational coordinate at which the Gaussian is centered. A very efficient and easy-to-use criterion is given for selecting which of these L² functions to include in the basis. Application to the three-dimensional H + H₂ reaction for various values of total angular momentum shows that accurate results can be obtained with half (or fewer) the number of basis functions that are necessary when asymptotic channel eigenfunctions are used for the internal degrees of freedom. The basis function selection criterion allows one to obtain moderately accurate results with very few basis functions and then to improve the accuracy systematically by increasing the basis in a determined fashion.