Comment on the interpretation of Hund's multiplicity rule

According to configuration interaction calculations of light atoms, ions, and molecules, the lower energy of the high-spin state arising from the same configuration is due to its greater electron-nucleus attraction, in contrast with the traditional explanation of Hund's rule which assumes a greater electron repulsion in the lower spin states. On the contrary, the electron is found to be greater in the high-spin state of every neutral species. Aiming at a systematic interpretation of Hund's rule, we here sketch a new method of performing accurate calculations for heavier atoms and molecules, which uses an orbital-dependent correlation energy functional based on density-functional theory.