Ideal-gas heat capacities are required to estimate caloric properties of fluids. Relationships between ideal-gas and molecular properties are given by statistical thermodynamics. For many molecules the required molecular data, i.e. vibrational frequencies, or heat capacity data from different references are contradictory or still lacking. Here a method is applied, which is able to predict the vibrational frequencies for many molecules from known data of few molecules with an accuracy sufficient for the calculation of heat capacities. A brief description of the method, based on a mechanical model of the molecules, is given. The application to all fluorochloro-substituted methanes will demonstrate its reliability. Reliable experimental heat capacity data indicate, that an accuracy of +/-0.8% is achieved.