Correlated ground-state ab initio calculations of polymethineimine

xAb initio restricted Hartree-Fock and various types of correlated calculat ions were performed on all three isomers of polymethineimine [CHN](infinity ) using 6-31G** basis sets. Starting from the Bloch-orbital-based Hartree-F ock results, the electron correlation contributions to the ground state ene rgies were calculated by considering finite clusters of increasing size mod elling the infinite system. Correlation methods such as Maller-Plesset seco nd-order perturbation theory and coupled-cluster theory with singles and do ubles, including a perturbative estimate of triples, were employed. At all levels of theory, and in contrast to the isoelectronic polyacetylene, the c is-transoid form of polymethineimine is found to be the lowest in energy. A Wannier-orbital-based and a Bloch-orbital-based Hartree-Fock approach have been tested for the equidistant all-trans form and the convergence of subs equent correlation calculations depending on the bond alternation and clust er size has been investigated. (C) 2000 Elsevier Science B.V. All rights re served.