Fundamental Understanding of Materials Properties based on the Exact Solution of Many Body Coulombic System byDiffusion Quantum Monte Carlo Method 

Yoshiyuki Kawazoe, Institute for Materials Research, Tohoku University, Japan

In 1925 Hund found his empirical rule on the maximum ground state spin state. 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. Total energy of the system is then E=T+V=-T=V/2.), kinetic energy and repulsive interactions between electrons are increased for the state.  Another impact of this study is that we clarify the fundamental reason of stability of materials.  Again when atoms gather to form molecules or crystals, energy gain is only achieved by the nucleus-electron attractive force, and electrons come closer to nucleus, the potential energy decreases, and the kinetic energy increases to realize stability of materials. This result proves that the Heitler-London model is invalid.