Near Hartree—Fock Calculations of the Force Constants and Dipole Moment Derivatives in Methane

The complete force field of methane is calculated from SCF—MO wavefunctions by applying the force method. Many different Gaussian basis sets are used, and the influence of different types of atomic functions on the force constants is discussed. The deviation of the calculated force constants from the observed values is similar for all treated basis sets. Therefore it is concluded that the deviation is mainly due to the neglect of correlation effects, and that there is little point in using very extended basis sets in Hartree-Fock calculations. The energy of our best calculation is estimated to be at most 0.01 a.u. above the HF limit. The results of this calculation are as follows (experimental values in brackets): (1) Force constants in millidynes per angstrom: F11=5.578(5.842), F22=0.537(0.486), F33=5.382(5.383), F34=0.202(0.206), F44=0.512(0.458); (2) Dipole moment derivatives in debyes per angstrom: μ3=−0.977 (± 0.833), μ4=−0.329 (± 0.373). The interpretation of the force field is discussed on the basis of a population analysis.