A refined estimate of the bond length of methane

The equilibrium structure of methane is determined by two different procedu res. The first is based on high level ab initio calculations using the CCSD (T) treatment of electron and basis sets containing up to 365 contracted Ga ussian functions. Using an extrapolation procedure to account for residual inadequacies of the basis set and an estimate of higher order correlation e ffects, an equilibrium internuclear separation of 1.0859(5) +/- 0.0003 Angs trom is obtained. This value is in almost perfect agreement with that obtai ned by correcting measuring ground-state rotational constants of methane is otopomers for vibrational effects and using the resulting empirical equilib rium rotational constants to infer the equilibrium structure.