Size-consistent self-consistent truncated or selected configuration interaction

Based on the principle of intermediate effective Hamiltonians, a simple procedure is proposed in order to eliminate the unlinked contributions of any truncated or selected configuration interaction (CI). The corrections are diagonal energy shifts, easily calculated. A self-consistent version is proposed, which insures separability if localized molecular orbitals (MO) are used. In the special case of double CI, the present method is an improved version of the coupled electron pair approximation (CEPA), but it may be applied to any selected model space, involving configurations of various degrees of excitation. The efficiency of the proposed algorithms is illustrated on a series of test calculations performed on Be2, F2, N2, and NH3.