Higher excitations for an exponential multireference wavefunction Ansatz and single-reference based multireference coupled cluster Ansatz:: Application to model systems H4, P4, and BeH2

This article reports on the convergence of the exponential multireference wavefunction Ansatz (MRexpT) [J. Chem. Phys. 123, 84102 (2005)] and the single-reference based multireference coupled cluster Ansatz [J. Chem. Phys. 94, 1229 (1991) [ with respect to higher cluster excitations. The approaches are applied to the H-4, P-4, and BeH2 model systems according to the recently published analysis by Evangelista et al. [J. Chem. Phys. 125, 154113 (2006)]. The results show both MRexpT and SRMRCC to be highly accurate although SRMRCC shows problems due to its lack of Fermi vacuum invariance (symmetry breaking). (C) 2008 American Institute of Physics.