Configuration interaction in a nonorthogonal determinant basis

A class of electron models which reduce to a superposition of nonorthogonal determinants is studied by methods of covariant quantum chemistry. A compact matrix representation is found which contains variational parameters only in the form of Fock-Dirac densities A, b of separate determinants ¦>,...,¦B>. Nonorthogonality of the determinants is taken into account by means of additional projectors obtained by pseudo-inversion of the products of the form AB-Orbital optimization is thus reduced to a clearly Hermitian eigenvalue problem. Inclusion in the initial model of configurations singly excited with respect to each of the determinants ¦a> ...,¦B> is discussed. This generalized Tamm-Dancoff model is studied in detail for the spin-symmetrized (half-projected) Hartree-Fock methods. Correlation effects in the excited states of model -systems of the alternant type are analyzed within the framework of the model.Translated from Teoreticheskaya i Éksperimentalnaya Khimiya, Vol. 22, No. 5, pp. 513–523, September–October, 1986.