On the use of MP2 theory for electron correlation in atoms and molecules

The use of Moeller-Plesset perturbation theory, frequently designated MPn, in describing electron correlation in atomic and molecular systems is critically examined. Some recently published studies have examined the higher terms in the Moeller-Plesset perturbation expansion and the convergence behaviour of the perturbation series. From these investigations conclusions about the applicability of the low order theory have been drawn which, in view of the widespread use of MP2, demand more thorough analysis. Combining the Rayleigh-Schrodinger and the Brillouin-Wigner perturbation expansions avoids an infinite order expansion and yields a closed expression which consists of the MP2 energy components together with a remainder term. The applicablity of MP2 theory then rests upon the magnitude of this remainder term rather than the behaviour of the higher order terms in the perturbation series.