Electron correlation theories and their application to the study of simple reaction potential surfaces

Abstract This paper serves two purposes. The first is to describe an implementation of the coupled cluster theory with double substitutions ( CCD ) previously developed by Cizek. The second is to apply this method and closely related fourth‐order perturbation methods to some simple molecules and reaction potential surfaces. These studies show that CCD theory gives results close to those of a Møller‐Plesset perturbation treatment to fourth order in the space of double and quadruple substitutions MP 4(DQ). Addition of contributions from single substitutions at fourth order makes little change in predicted relative energies. Preliminary results on the potential surfaces for 1,2‐hydrogen shifts in C 2 H 2 , HCN, CH 2 O, and N 2 H 2 are discussed and compared with previous studies.