Self-consistent perturbation theory for interacting electron systems

A general self-consistent perturbation formalism is developed for two interacting closed-shell molecules. The zero-order many-electron wavefunction of the coupled system is represented by an antisymmetrized product of the mutually orthogonal single-determinant wavefunctions of the constituent parts. The method involves the application of perturbation theory directly to the Fock-Dirac density matrix and thus avoids the calculation of perturbed orbitals. Self-consistency to first order is maintained by an iterative determination of the matrix elements of the Hartree-Fock perturbation operator. Explicit expressions are given for the matrix elements of the one-electron and two-electron perturbations and for the energy changes to first and second order.