ON THE EVALUATION OF NONORTHOGONAL MATRIX-ELEMENTS

Lowdin's formula for non-orthogonal matrix elements is derived and elucidated. Algorithms for cofactor evaluation are discussed. It is shown that for matrix elements involving two-electron operators the effort associated with the computation of the cofactors is only about three floating point operations per cofactor. We conclude that the non-orthogonality problem does not reside in the evaluation of the cofactors and that in situ generation is a viable approach. Explicit algorithms are given.