Correlated ground-state ab initio calculations of polymethineimine

Abstract

Ab initio restricted Hartree–Fock and various types of correlated calculations were performed on all three isomers of polymethineimine [CHN] ∞ using 6-31G ** basis sets. Starting from the Bloch-orbital-based Hartree–Fock results, the electron correlation contributions to the ground state energies were calculated by considering finite clusters of increasing size modelling the infinite system. Correlation methods such as M øller–Plesset second-order perturbation theory and coupled-cluster theory with singles and doubles, including a perturbative estimate of triples, were employed. At all levels of theory, and in contrast to the isoelectronic polyacetylene, the cis-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 subsequent correlation calculations depending on the bond alternation and cluster size has been investigated.