Force constants for molecules with secular equations of order n > 2 have been determined with the help of various approximation methods based on the General Valence Force Field. The applicability of the approximations have been tested empirically. It cannot be predicted with certainty whether any approximation method yields reliable force constants. Some methods have been found to be inapplicable. The calculation of off-diagonal force constants with any approximation method is impossible. The principle of the quasi-determination of off-diagonal force constant matrix elements from the form of the G matrix on which most approximation methods are based, is critically discussed. It is shown that diagonal force constants calculated with approximation methods are more reliable if the coupling between the vibrations in the same species is low. The application of approximation methods for the determination of force constants without critical consideration is not meaningful.
