The gauche-gauche (g,g) conformation of the dimethyl phosphate anion serves as a simple model for the phosphodiester group of nucleic acids. Detailed knowledge of the normal modes of vibration of dimethyl phosphate and other phosphodiester analogues is expected to provide a foundation for understanding the conformation-sensitive bands in vibrational spectra of DNA and RNA. In the previous paper of this series (Guan, Y.; et al. Biophys. J. 1994, 66, 225-235), infrared and Raman spectra of the g,g conformer of dimethyl phosphate [(CH3O)(2)PO2-], including deuterium [(CD3O)(2)PO2-] and carbon-13 [((CH3O)-C-13)(2)PO2-] isotopomers, were reported and assigned in conjunction with a complete normal coordinate analysis. The normal mode calculations were accomplished by use of a generalized valence force field. Here, we demonstrate that an ab initio molecular force field for the g,g conformer of dimethyl phosphate, computed using the 3-21+G* basis set, is consistent with the empirically based valence force field reported previously. We have obtained the force field scaling factors which produce satisfactory agreement between the calculated and experimental vibrational frequencies of each of the three dimethyl phosphate isotopomers in the g,g conformation. Ab initio normal mode assignments, in terms of the potential energy distribution, are in general accord with those obtained from the normal coordinate analysis. Agreement between the ab initio and normal coordinate results supports the reliability of the empirically derived force field for extension to other phosphodiester conformations of dimethyl phosphate, as well as to more complex diester derivatives of the nucleic acid orthophosphate group.