Ab Initio Extended Density Functional Theory for Strongly Correlated Electron Systems: Fundamental Aspects of the Broken-Symmetry Approach and Possible Applications for Molecular Material Design

Abstract

Abstract We review the extended density functional theory (DFT), in which we make full use of the broken-symmetry feature of the Kohn–Sham (KS) DFT. The theory is based on the classification of broken-symmetry solutions of the Hartree–Fock theory, which has been developed by some precursors in the field “broken-symmetry quantum chemistry”. We describe the fundamental characteristics of the DFT based on generalized spin orbitals (GSOs) in relation to the extension of the constrained search region of DFT. In addition, some important applications, such as noncollinear magnetism of multicenter radicals and radical dissociation followed by spin rotations that are firstly enabled by the use of the GSO-DFT, are presented. Furthermore, future directions covering quantum fluctuations between several collective modes, ab initio molecular dynamics, and relativistic phenomena are also described from the viewpoint of search regions of the extended DFT.