STATE-SELECTIVE MULTIREFERENCE COUPLED-CLUSTER THEORY EMPLOYING THE SINGLE-REFERENCE FORMALISM - IMPLEMENTATION AND APPLICATION TO THE H8 MODEL SYSTEM

The new state-selective (SS) multireference (MR) coupled-cluster (CC) method exploiting the single-reference (SR) particle-hole formalism, w hich we have introduced in our recent paper [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)], has been implemente d and the results of the pilot calculations for the minimum basis-set (MBS) model composed of eight hydrogen atoms in various geometrical ar rangements are presented. This model enables a continuous transition b etween degenerate and nondegenerate regimes. Comparison is made with t he results of SR CC calculations involving double (CCD), single and do uble (CCSD), single, double, and triple (CCSDT), and single, double, t riple, and quadruple (CCSDTQ) excitations. Our SS CC energies are also compared with the results of the Hilbert space, state-universal (SU) MR CC(S)D calculations, as well as with the MR configuration interacti on (CI) results (with and without Davidson-type corrections) and the e xact correlation energies obtained using the full CI (FCI) method. Alo ng with the ground-state energies, we also analyze the resulting wave functions by examining some selected cluster components. This analysis enables us to assess the quality of the resulting wave functions. Our SS CC theory truncated at double excitations, which emerges through s election of the most essential clusters appearing in the full SR CCSDT Q formalism [SS CCSD (TQ) method] provides equally good results in non degenerate and quasidegenerate regions. The difference between the gro und-state energy obtained with the SS CCSD(TQ) approach and the FCI en ergy does not exceed 1.1 mhartree over all the geometries considered. This value compares favorably with the maximum difference of 2.8 mhart ree between the SU CCSD energies and the FCI energies obtained for the same range of geometries. The SS CCSD(T) method, emerging from the SR CCSDT theory through selection of the most essential clusters, is les s stable, since it neglects very important semi-internal quadruple exc itations. Unlike the genuine multideterminantal SU CC formalism, our S S CC approach is not affected by the intruder state problem and its co nvergence remains satisfactory in nondegenerate and quasidegenerate re gimes.