NONITERATIVE 5TH-ORDER TRIPLE AND QUADRUPLE EXCITATION-ENERGY CORRECTIONS IN CORRELATED METHODS

In critical cases, single-reference correlated methods like coupled-cluster theory or its quadratic CI approximations fail because of the importance of additional highly excited excitations that cannot usually be included, like connected triple and quadruple excitations. Here we present the first, non-iterative method to evaluate the full set of fifth-order corrections to CCSD and QCISD and assess their accuracy compared to full CI for the very sensitive Be2 curve and other cases. © 1990.