The reason why these vibrations are not absorbing their expected fraction of heat energy input is provided by quantum mechanics. According to that theory, the energy stored in each degree of freedom must increase or decrease only in certain amounts (quanta). Therefore, if the temperature T of the system is not high enough, the average energy that would be available for some of the theoretical degrees of freedom (kT / f) may be less than the corresponding minimum quantum. If the temperature is low enough, that may be the case for practically all molecules. One then says that those degrees of freedom are "frozen". The molar heat capacity of the gas will then be determined only by the "active" degrees of freedom — that, for most molecules, can receive enough energy to overcome that quantum threshold.