Coupled-Cluster Methods That Include Connected Quadruple Excitations, T4: CCSDTQ-1 and Q(CCSDT)

摘要

Several coupled-cluster methods that include the connected, T4 contribution have been formulated and implemented. All are correct through the fifth-order energy. CCSDTQ-1 reproduces the full CI results for BH, FH AND H2O to a mean absolute error of 0.36 mhartree, compared to 0.93 mhartree for CCSDT. Based on CCSDTQ-1, some noniterative methods for the inclusion of quadruples have also been considered. One of them, denoted as Q(CCSDT), has an average error of 0.23 mhartree and has only an N(sup 7) basis set dependence. This method offers a convenient estimate of the principal effects due to T4. Reprints. (JHD)