The Theoretical Study of Hund's Rule
Ni Shenkuan (Department of Chemistry)
The traditional explanation for Hund's rules uses first order perturbation theory and assumes that a singlet minus triplet energy difference △E equals the difference in electron repulsion energy △(Vce), namely, The lowering of the total energy is ontirely due to the lowering of the electron repulsion energy. This is a misinterpretation. Actually, due to the contraction of the orbital, the state with lower energy in a multiplet has the larger electron repulsion energy but also by far the most negative electron-nuclear attraction enery. It is the electron nuclear attraction that dominates the energy behaviour in a multiplet and not the electron repulsion.