A highly accurate ab initio potential energy surface for methane

Alec Owens, Sergei N. Yurchenko, Andrey Yachmenev, Jonathan Tennyson, Walter Thiel
2016 Journal of Chemical Physics  
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
more » ... tion. Sub-wavenumber accuracy is achieved for the majority of experimentally known vibrational energy levels with the four fundamentals of ^12CH_4 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.
doi:10.1063/1.4962261 pmid:27634258 fatcat:ohrjvmvxezhzjam4ouk2oe7qqy