The Approximate Inclusion of Triple Excitations in EOM-type Quantum Chemical Methods

In non-relativistic quantum mechanics, stationary states of molecules and atoms are described by eigenvectors of the Hamiltonian operator. For one-electron systems, such as the hydrogen atom, the solution of the eigenvalue problem (Schro ̈dinger’s equation) is straightforward, and the results show excellent agreement with experiment. Despite this success, the multi electron problem corresponding to virtually every system of interest in chemistry has resisted attempts at exact solution. Perhaps the most popular method for obtaining approximate, yet very accurate results for the ground states of molecules is the coupled cluster approximation. Coupled cluster methods move beyond the simple, average field Hartree-Fock approximation by including the effects of excited configurations generated in a size consistent manner. In this paper, the coupled cluster approximation is developed from first principles. Diagrammatic methods are introduced which permit the rapid calculation of matrix elements appearing in the coupled cluster equations, along with a systematic approach for unambiguously determining all necessary diagrams. A simple error bound is obtained for the ground state energy by considering the coupled cluster equations as entries in the first column of a matrix whose eigenvalues are the exact eigenvalues of the Hamiltonian. In addition, a strategy is considered for treating the error in the ground state energy perturbatively