Ability of a coupled electron pair approximation to treat single bond breakings

The self-consistent size-consistent CI ((SC)2CI), which considers all exclusion principle violating (EPV) diagrams, is able to treat single bond breakings, despite its single-reference character. It is shown here that a CEPA-3 (coupled electron pair approximation) dressing, considering only the EPVs through the occupied molecular orbitals, treats satisfactorily the single bond breaking processes, while the uniform dressings, e.g. CEPA-0, the averaged coupled pair functional (ACPF) or averaged quadratic coupled cluster (AQCC) lead to non-convergence or spurious energy lowering in such processes. A number of test calculations illustrate this rationalizable feature.