Harmonic force fields were calculated using fourth-order Moller-Plesset (MP4) theory for CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, acetylene, HCN, ethylene, formaldehyde, chloromethane, and fluoromethane. These were compared with the experimental force fields. Most of the diagonal force constants for these molecules are computed reliably (1-2% error) at the MP4/6-311+g(2d,2p) level. Exceptions are force constants for out-of-plane bending in the ct-bonded molecules and for stretching of the multiple bonds. For computations using second-order Moller-Plesset (MP2) theory, the 6-31g* basis is inferior to basis sets having polarization functions on all atoms such as 6-311+g** and 6-311+g(2d,2p). The force fields computed with MP2 theory are as accurate as those computed at the MP4 level except that HX stretching force constants are consistently overestimated by ca. 3% with MP2 theory. Reduced isotopic partition function ratios, (S-2/S-1)f, and fractionation factors (FFs) for hydrogen/deuterium substitution were computed for the same set of molecules. The (s(2)/s(1))f values were compared with values computed from experimental harmonic frequencies. The (s(2)/s(1))f values are overestimated by RHF and MP2 theory, but the error is removed when frequencies for the isotopomers are scaled uniformly. The resulting values agree with experiment to within +/-2.5% at the MP2 level and +/-3.3% at the RHF level.