DFT+U Type Strong Correlation Functional Derived from Multiconfigurational Wavefunction Theory

We present a DFT+U-type functional for strong correlation, derived from multiconfigurational wavefunction theory. The reference system experiences electron-electron interactions only in DFT+U-type atomic states, yielding a block-localized configuration interaction Hamiltonian which depends on the atomic state occupancies and the promotion energies of doubly excited determinants. Simple approximations for the promotion energies recover the flat-plane condition and provide beyond-zero-sum accuracy for iron spin-crossover complexes.