title: Molecular mechanics-valence bond method for planar conjugated hydrocarbon cations.
abstract: We present an extension of the molecular mechanics-valence bond (MMVB) hybrid method to study ground and excited states of planar conjugated hydrocarbon cations. Currently, accurate excited state calculations on these systems are limited to expensive ab initio studies of smaller systems: up to 15 active electrons in 16 pi orbitals with complete active space self-consistent field (CASSCF) theory using high symmetry. The new MMVB extension provides a faster, cheaper treatment to investigate larger cation systems with more than 24 active orbitals. Extension requires both new matrix elements and new parameters: In this paper we present both, for the limited planar case. The scheme is tested for the planar radical cations of benzene, naphthalene, anthracene, and phenanthrene. Calculated MMVB relative energies are in good agreement with CASSCF results for equilibrium geometries on the ground and first excited states, and conical intersections.
authors.affiliation: Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom.
authors.forename: Katherine F
authors.initials: KF
authors.lastname: Hall
authors.forename: Andrei M
authors.initials: AM
authors.lastname: Tokmachev
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Bearpark
authors.forename: Martial
authors.initials: M
authors.lastname: Boggio-Pasqua
authors.forename: Michael A
authors.initials: MA
authors.lastname: Robb
mesh_terms: D000465:Algorithms; D002412:Cations / Q000737:chemistry*; D003198:Computer Simulation; D006838:Hydrocarbons / Q000737:chemistry*; D019563:Mechanics; D008956:Models, Chemical; D008958:Models, Molecular
chemical_list: D002412:Cations; D006838:Hydrocarbons