The simple harmonic force constants for a valence force field have been computed using Frost's Floating Spherical Gaussian Orbital (FSGO) method for the tetrahedral molecules BH^–₄, CH₄ and NH⁺₄ and comparison has been made with the experimental values. In general the agreement is good for BH^–₄, less good for CH₄ and poor for NH⁺₄. The calculated main force constants are larger than the experimental ones. The signs of the bond–bond, angle–angle and bond–angle cross-term constants are discussed. It is found that the signs correspond to the change in geometry expected on dissociation to two hydrogens and a triatomic species. This is true experimentally without exception and theoretically with one possible exception. However, dissociation products expected theoretically are not the same as those that would, in fact, be formed. This accounts for the differences between theory and experiment in this respect in almost all instances. It suggests that an open-shell calculation is required.