THE ACCELERATION OF CONVERGENCE OF MANY-BODY PERTURBATION-THEORY - UNLINKED-GRAPH SHIFT IN MOLLER-PLESSET PERTURBATION-THEORY

We recently proposed (parts I and II of this series of publications) a new method of infinite subsummation in the Moller-Plesset perturbation series with the objective of ''effecting convergence of otherwise divergent series''. In the present work we undertake to study the importance of a shift of unlinked graphs in the Moller-Plesset renormalization term. We demonstrate that this modification of many-body perturbation theory presents a viable starting point for new approximations devised to obtain a substantial fraction of the correlation energy at low order. We present an explicit investigation of the excited states of LiH in a suitable full-CI model space, using our method to calculate the correlation energy and monitoring convergence up to tenth order.