Coupled alkali halide color centers: Fractional charge errors, fractional spin errors, and a failure of spin symmetry breaking produce challenging tests for condensed-phase electronic structure calculations

Abstract

Color centers (F-centers) consist of defect-trapped electrons confined and shielded by a surrounding ionic lattice. This work shows how adjacent color centers in lithium fluoride provide a suite of challenging tests for electronic structure calculations in condensed phases, mimicking theoretically well-studied but experimentally fleeting gas-phase model systems such as stretched H2+, stretched H2, and stretched H3+. Singlet-triplet gaps and electron transfer reactions among these centers exhibit delocalization (fractional charge), strong left-right correlation (fractional spin), and a density-driven failure of spin-symmetry-broken singlet calculations. Tests of representative density functional theory approximations show that new “non-zero-sum” approaches qualitatively improve agreement with correlated multireference benchmark values.