Numerical techniques for the evaluation of non-adiabatic interactions and the generation of quasi-diabatic potential energy surfaces using configuration interaction methods

CI techniques, based on nearly complete expansions over determinants for su bsets of molecular orbitals and electrons, have been extended to evaluate, by finite differences, non-adiabatic interactions arising from the nuclear kinetic energy operator. A functional of the non-adiabatic interactions is defined and, from the minimum condition of the functional, a simple express ion is obtained for the transformation matrix to the quasi-diabatic basis. Two different schemes for the minimization of the functional, with and with out constraints to the form of the transformation matrix, have been compare d. Simple tests are presented to verify the numerical accuracy of the non-a diabatic interactions and to compare the two schemes for the minimization o f the functional.