Hund found his empirical rule in 1925 as “Minimum energy state in atoms and molecules with the same electron configuration defined by total orbital and spin angular momentum exhibits the maximum spin multiplicity”. Immediately after that quantum theoretical studies have been conducted and Slater explained this rule in 1929 by a perturbative approach based on the electron exchange energy. In most of the popular textbooks today still use this traditional explanation. However, Slater’s explanation has been doubted theoretically by Davidson, Boyd, and other researchers by using Hartree-Fock level and better approximations. We have solved the many-body Shroedinger equation exactly with diffusion quantum Monte Carlo method (DQMC) and finally confirmed that the traditional Slater’s explanation is invalid. The exact reason of the Hund’s multiplicity rule is that the energy gain for maximum spin state is realized mainly by the electron-nucleus attractive interaction. Based on the virial theorem (2T+V=0), kinetic energy and repulsive interactions between electrons are increased for the state. The first paper published in 2004 in Journal of Chemical Physics on the ground state in carbon atom by Hongo and Yasuhara has attracted strong attention. Recent results on the first and second period by Oyamada et al. published in the same journal proved that our conclusion is correct for all the atom species in these rows.
Prof. Kawazoe group（Materials Design by Computer Simulation）