Ideal gas heat capacities are important for the calculation of caloric properties of real fluids. But as shown in the past, they are not always available with required accuracy or are still lacking. In this paper, we combine quantum mechanical calculations and statistical thermodynamics. In order to find a route to a reliable prediction of ideal gas heat capacity data, we applied various quantum mechanical methods differing by computational effort to difluoromethane (CH2F2). Only the structural formula and fundamental physical constants enter into the calculations. It is shown that quantum mechanics leads to accurate molecular data. Reliable experimental heat capacity data reveal that an accuracy of better than 0.5% is obtained for the ideal gas heat capacity of difluoromethane.