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_NH_N (N = 6, 10, 14, 18) in the Pariser–Parr–Pople (PPP) approximation, as a function of the hopping parameter β. A wide range of values of β is considered, from a highly correlated situation, β = 0, to a very weakly correlated limit, β = −10 eV. An estimate of the role of higher than two‐body connected cluster components was done, through a partial (i.e., limited to two‐body terms in the exponent) cluster analysis performed on the FCI wave function. The comparison with the approximate coupled pair theory that accounts for quadruply excited clusters [see P. Piecuch and J. Paldus, Theor. Chim. Acta 78, 65 (1990)] shows a good agreement in the whole range of the hopping parameter, particularly when the contribution of connected triples excitations is also taken into account. © 1994 John Wiley & Sons, Inc.