Exchange and correlation in open systems of fluctuating electron number
Exchange and correlation in open systems of fluctuating electron number
While the exact total energy of a separated open system varies linearly as a function of average electron number between adjacent integers, the energy predicted by semilocal density-functional approximations is concave up and the exact-exchange-only or Hartree-Fock energy is concave down. As a result, semilocal density functionals fail for separated …