Frozen-density embedding as a quasi-diabatization tool: Charge-localized states for spin-density calculations.

We present an effective approach for (spin-)density calculations of open-shell molecular complexes that avoid both an overdelocalization of spin densities as often observed in approximate Kohn-Sham-density functional theory (KS-DFT) calculations and an overlocalization of spin densities as may occur in fragment approaches with non-suitable fragment choices. The method is based on the frozen-density embedding formalism and makes use of non-orthogonal, spin-/charge-localized Slater determinants, which provides a basis for qualitatively correct descriptions of intersystem spin-density delocalization. The reliability of this method is tested on four complexes featuring different molecular sizes and interactions and showing different degrees of spin-density delocalization, ranging from fully localized to fully delocalized. The resulting spin densities are compared to accurate ab initio results. The method is clearly more robust than the corresponding KS-DFT approximations, as it works qualitatively correct in all cases studied.