Tuning strong correlation and charge transfer competition using fractional nuclear charge: emergence of exotic metal-insulator transitions in a 1D hydrogen-like chain*

In chemistry and condensed matter physics the solution of simple paradigm systems, such as the hydrogen atom and the uniform electron gas, plays a critical role in understanding electron behaviors and developing electronic structure methods. The H2 molecule is a paradigm system for strong correlation with a spin-singlet ground state that localizes the two electrons onto opposite protons at dissociation. Here, we extend H2 to a new paradigm system by using fractional nuclear charges (FNC) to break the left-right nuclear symmetry, thereby enabling the competition between strong correlation and charge transfer. We use H2_FNC to connect the self-interaction and strong correlation errors of common density functional approximations, and show how a functional's accuracy for H2_FNC is reflected in its accuracy for transition metal monoxides. We further show the competition drive exotic metal-insulator transitions in a 1D FNC hydrogen chain at large nuclear separations. The extension therefore lays a foundation for improving practical electronic structure theories and provides a playground for analyzing how the competition appears and evolves with exotic properties.