Electron–electron correlation energy calculations by superposition of nonorthogonal Slater determinants

A tractable calculation algorithm to calculate essentially exact electron–electron correlation energies of few-electron systems by the superposition of nonorthogonal Slater determinants (SDs) is presented. The key to the proposed procedure for updating nonorthogonal one-electron wavefunctions to ground states is that linearly independent multiple correction functions are employed and optimized on the basis of a variational principle. The accuracy and applicability of the present scheme are demonstrated through calculations of the ground-state energies for atoms and molecules. The convergence performance to the ground state is improved by multiplying the correction functions. A drastic reduction of the number of SDs required to determine the ground-state energies and a modest increase with increasing system size compared with the full configuration interaction (CI) method are also demonstrated.