text: Classically, the kinetic energy of rotation is
  :where
  :: is the angular momentum
  :: is the moment of inertia of the molecule
  For microscopic, atomic-level systems like a molecule, angular momentum can only have specific discrete values given by
  :where  is a non-negative integer and  is the reduced Planck constant.
  Also, for a diatomic molecule the moment of inertia is
  :where
  :: is the reduced mass of the molecule and
  :: is the average distance between the centers of the two atoms in the molecule.
  So, substituting the angular momentum and moment of inertia into E, the rotational energy levels of a diatomic molecule are given by: