Harmonic force constants and frequencies have been calculated for the molecules H4SiO4 and SiF4 by using STO-3G, 3-21G, 6-31G, and 6-31G* basis sets at the SCF level. Results for SiF4 are compared with experimental values from the literature corrected for anharmonicity. Internal symmetry force constants for SiF4 at the 6-31G* basis level are F11 = 806.0, F22 = 28.02, F33 = 722.5, F34 = 24.67, and F44 = 48.11 N m-1, in good agreement with experiment. The minimum energy geometry of H4SiO4 has previously been suggested to be a conformation with D2d symmetry. The present study indicates that the D2d structure is unstable with respect to rotation of the H atoms about the Si-O bonds and is the geometry of a transition state rather than that adopted at the global minimum. Internal symmetry force constants and harmonic frequencies for H4SiO4 are compared as a function of basis set. Selected internal force constants have been extracted for comparison with previous estimates from empirical force constant calculations for silicates. The present work suggests that Hartree-Fock calculations can provide realistic force constants for silicate molecules and clusters. © 1986 American Chemical Society.