SIZE-CONSISTENT SELF-CONSISTENT TRUNCATED OR SELECTED CONFIGURATION-INTERACTION

Based on the principle of intermediate effective Hamiltonians, a simpl e procedure is PrOPOsed in order to eliminate the unlinked contributio ns of any truncated or selected configuration interaction (CI). The co rrections are diagonal energy shifts, easily calculated. A self-consis tent version is proposed, which insures separability if localized mole cular orbitals (MO) are used. In the special case of double CI, the pr esent method is an improved version of the coupled electron pair appro ximation (CEPA), but it may be applied to any selected model space, in volving configurations of various degrees of excitation. The efficienc y of the proposed algorithms is illustrated on a series of test calcul ations performed on Be2, F2, N2, and NH3.