DIAGRAMMATIC ANALYSIS AND APPLICATION OF THE COUPLED-CLUSTER RESPONSEAPPROACH TO GROUND-STATE EXPECTATION VALUES

The response approach to couple cluster (CC) expectation values devise d by Monkhorst and the normal coupled cluster method (NCCM) discussed by Arponen and Bishop are considered from a diagrammatic point of view . The perturbative diagrammatic content of the operator that parametri zes the NCCM bra state is discussed in detail. The method is applied t o the calculation of the one-particle reduced density matrix of the Be atom and the HF molecule for different values of the internuclear dis tance. Various contributions to the total energy obtained in the NCCM framework are compared to results from accurate multireference configu ration interaction calculations. Such a comparison is shown to provide a much more stringent test on the accuracy of the CC formalism than w ill a comparison of total energy alone.