Cubic force constants and equilibrium geometry of methane from Hartree–Fock and correlated wavefunctions

The ab initio harmonic and cubic force field of methane has been determined, both from Hartree–Fock and from highly correlated CEPA wavefunctions. The effect of correlation is small on the force field, particularly on the cubic constants. The ab initio cubic force constants were used to calculate the equilibrium CH bond length from the electron diffraction rg and from the spectroscopic r0 values. The weighted mean of the results is re=1.0862 ±0.0005 Å. There remains a small but disturbing discrepancy between the best ab initio and the present corrected experimental results, both for the bond length and for the CH stretching force constants.