title: A highly accurate ab initio potential energy surface for methane.
abstract: A new nine-dimensional potential energy surface (PES) for methane has been generated using state-of-the-art ab initio theory. The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit and incorporates a range of higher-level additive energy corrections. These include core-valence electron correlation, higher-order coupled cluster terms beyond perturbative triples, scalar relativistic effects, and the diagonal Born-Oppenheimer correction. Sub-wavenumber accuracy is achieved for the majority of experimentally known vibrational energy levels with the four fundamentals of (12)CH4 reproduced with a root-mean-square error of 0.70 cm(-1). The computed ab initio equilibrium C-H bond length is in excellent agreement with previous values despite pure rotational energies displaying minor systematic errors as J (rotational excitation) increases. It is shown that these errors can be significantly reduced by adjusting the equilibrium geometry. The PES represents the most accurate ab initio surface to date and will serve as a good starting point for empirical refinement.
authors.affiliation: Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, United Kingdom.
authors.forename: Alec
authors.initials: A
authors.lastname: Owens
authors.affiliation: Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, United Kingdom.
authors.forename: Sergei N
authors.initials: SN
authors.lastname: Yurchenko
authors.affiliation: Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, United Kingdom.
authors.forename: Andrey
authors.initials: A
authors.lastname: Yachmenev
authors.affiliation: Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, United Kingdom.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Tennyson
authors.affiliation: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.
authors.forename: Walter
authors.initials: W
authors.lastname: Thiel