Collect. Czech. Chem. Commun. 1988, 53, 1919-1942

Coupled-pair theories and Davidson-type corrections for quasidegenerate states: The H4 model revisited

Josef Paldusa,b,c,d, Paul E. S. Wormera,c and Marc Benardd

a Department of Applied Mathematics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
b Institute for Advanced Study Berlin, D-1000 Berlin 33, F.R.G.
c Institute for Theoretical Chemistry, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands
d Laboratoire de Chimie Quantique, E.R. 139 du C.N.R.S., Institut Le Bel, F-67000 Strasbourg, France


The performance of various variational and non-variational approaches to the many-electron correlation problem is examined for a simple four-electron model system consisting of two stretched hydrogen molecules in trapezoidal, rectangular and linear configurations, in which the degree of quasi-degeneracy can be continuously varied from a non-degenerate to an almost degenerate situation. In contrast to an earlier work (K. Jankowski and J. Paldus, Int. J. Quantum Chem. 18, 1243 (1980)) we employ a double-zeta plus polarization basis and examine both single reference and multireference configuration interaction and coupled-cluster-type approaches. The performance of various Davidson-type corrections is also investigated.