A COMPARISON OF THE EFFICIENCY AND ACCURACY OF THE QUADRATIC CONFIGURATION-INTERACTION (QCISD), COUPLED CLUSTER (CCSD), AND BRUECKNER COUPLED CLUSTER (BCCD) METHODS

The coupled-cluster method restricted to single and double excitations from a closed-shell reference function (CCSD) and the corresponding quadratic configuration interaction method (QCISD) are formulated in terms of quantities which can be computed directly from the two-electron integrals in AO basis. A simple yet effective method to accelerate convergence in Brueckner coupled-cluster (BCCD) calculations is also described. Using this procedure BCCD calculations require no more effort than CCSD calculations. In order to compare the accuracy of all three methods potential energy functions and spectroscopic constants have been calculated for N2, CO, F2, and HF using large basis sets.