Approximate fourth‐order perturbation theory of the electron correlation energy

Abstract An approximate fourth‐order expression for the electron correlation energy in the Møller–Plesset perturbation scheme is proposed. It takes into account all the contributions to the fourthorder energy neglecting only those of the triple‐substituted determinants. It is size consistent and correct to fourth order for an assembly of isolated two‐electron systems. Illustrative calculations are reported for a series of small molecules.