An electron pair operator approach to coupled cluster wave functions. Application to He2, Be2, and Mg2 and comparison with CEPA methods

A method for obtaining coupled cluster expansions with double substitutions (CCD) utilizing the electron pair operator approach of self-consistent electron pair (SCEP) theory is presented. A fairly simple operator is developed in this method and its calculation increases the expense over a typical variational configuration expansion only moderately. With this method, large basis set calculations have been performed on the weakly interacting dimers He2, Be2, and Mg2. Comparison calculations have been performed with various types of coupled electron pair approximations (CEPA), which may be viewed as approximations to coupled cluster theory, and with modification of the internal orbitals in the CCD treatment.