Calculations of the force field of the methane molecule

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Abstract
SCF and CAS SCF calculations have been carried out to determine the quadratic and some of the higher force constants of the methane molecule. The calculated SCF value of r e is 1·08258 Å which is very close to Gray and Robiette's experimental value of 1·0858 (±0·0010) Å. Correlation contributions to most of the quadratic constants are significant and are largely recovered by the CAS SCF calculations. The thirty-three independent quartic symmetry co-ordinate force constants are given explicitly for the first time together with additional, dependent quartic constants and some of them calculated at the SCF level.