In classical mechanics, a molecule can be seen as a collection of M nuclei and N electrons. Therefore, the system of M + N particles has 3(N + M) degrees of freedom to describe its motions. First, one can fix in space the location of the heavy nuclei (fixed nuclei approximation). The symmetry of this spatial distribution of nuclei can be associated with a 'molecular point group', which is a symmetry group corresponding to a fixed point [the center of mass (CM)]. The 3N degrees of freedom describe the motion of the electrons around the frozen frame, and the corresponding energy of motion is the electronic energy E e . We can regroup the nuclei and electrons into 3M effective atoms, and fix the origin of the system of coordinates in the CM of the molecule. The motion of this point in space is described by three degrees of freedom, and gives the translational energy of the molecule that is directly related to thermal energy. According to the equipartition principle, the energy is 3/2kT, where k is the Boltzmann constant. For 1 mol of molecules, we multiply by Avogadro's number, N A , and k is simply replaced by N A k = R, the gas constant,