Correct interpretation of Hund's multiplicity rule for atoms and molecules

A diffusion Monte Carlo study of the second and third row atoms (C, N, O, Si, P, and S) is performed to give the correct interpretation of Hund's multiplicity rule. The stability of the highest spin-multiplicity state relative to the next highest spin-multiplicity state for all the atoms is ascribed to the greater electron-nucleus attraction energy that is gained at the cost of increasing the electron-electron repulsion energy as well as the kinetic energy. By means of complete active space SCF, we have also ascertained that the same interpretation applies to the methylene molecule CH₂.