Correlated ground-state ab initio calculations of polymethineimine Original Research Article

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 Møller–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.