9 Many - Body Perturbation Theory and its Application to the Molecular Structure Problem

This report covers developments in the theory and application of many-body perturbation theory to molecular systems during the period June 2003 through to May 2005. It thus continues three earlier reviews in this series which, in turn, built on my report, written about twenty-five years ago, for a previous Specialist Periodical Reports series published in 1981. Applications of many-body perturbation theory to the molecular electronic structure problem have been published during the reporting period in an ever increasing range of scientific areas. In particular, in its second order form, many-body perturbation theory continues to be the most widely used ab initio quantum chemical method for describing the effects of electron correlation. A review of the numerous applications reported during the period under consideration is given in Section 4. Writing in 2005, the centenary of the three famous publications by Albert Einstein which marked a turning point in early twentieth century science, it seemed appropriate in this review to consider some of the major trends in early twenty-first century molecular science and the contribution of many-body perturbation theory to their study. In the introduction to his Lectures on Physics, Richard P. Feynman asserts that the atomic theory of matter is the most important of scientific theories since it underpins our explanation of the material world. He wrote