Equipartition theorem

The Theorem of the Equipartition of energy is a construct of classical mechanics and was first introduced by James Clerk Maxwell (1831-1879). It sates:

"Every kind of molecule has a particular number of degrees of freedom f which are independent ways in which it can store energy. Each such degree of freedom has associated with it, on average, an energy of ${\frac {1}{2}}\ kT$ per molecule or ${\frac {1}{2}}\ RT$ per mole, where k is the Boltzmann constant, R is the molar gas constant and T is temperature in kelvin.".

Degrees of freedom can be translational, rotational, or oscillatory.

Simple particles such as a monoatomic gas for instance will have three degrees of freedom, one for each dimension of potential movement. More complex molecules can have more degrees of freedom.