FULL CONFIGURATION-INTERACTION STUDY OF THE GROUND-STATE OF CLOSED-SHELL CYCLIC PPP POLYENES

Full configuration interaction (FCI) calculations are reported for the closed-shell cyclic polyenes C(H)H(N) (N = 6, 10, 14, 18) in the Pari ser-Parr-Pople (PPP) approximation, as a function of the hopping param eter beta. A wide range of values of beta is considered, from a highly correlated situations, beta = 0, to a very weakly correlated limit, b eta = -10 eV. An estimate of the role of higher than two-body connecte d cluster components was done, through a partial (i.e., limited to two -body terms in the exponent) cluster analysis performed on the FCI wav e function. The comparison with the approximate coupled pair theory th at accounts for quadruply excited clusters [see P. Piecuch and J. Pald us, Theor. Chim. Acta 78 65 (1990)] shows a good agreement in the whol e range of the hopping parameter, particularly when the contribution o f connected triples excitations is also taken into account. (C) 1994 J ohn Wiley & Sons, Inc.