A b i n i t i o computation of force constants. The second and third period hydrides

The force method has been employed to calculate the force constants of HF, H2O, NH3, CH4, HCl, H2S, PH3, and SiH4. The computations were performed using wavefunctions generated by both the STO-3G and 4-31G basis sets that are standard in the Gaussian 70 program system. The 4-31G basis set provides reliable harmonic force constants, but those computed with the STO-3G basis set are poor. Cubic force constants computed at the 4-31G level are fair to very good. Cubic stretching force constants are reproduced especially well, regardless of the basis set. Quartic stretching force constants are good, but quartic bending constants are in error. A component analysis has been developed and relations have been found that may be useful for qualitative and quantitative discussions.