Towards the Description of Covalent Bonds in Subsystem Density-Functional Theory

Current applications of frozen-density embedding (FDE)—or more generally subsystem density-functional theory (DFT) schemes—are limited to subsystems that are not connected by covalent bonds. This restriction is due to the insufficiencies of the available approximate kinetic-energy functionals, which are used to calculate the contribution of the nonadditive kinetic energy to the effective embedding potential. In this Chapter, we discuss two different approaches to overcome these limitations and to extend the applicability of the FDE scheme to subsystems connected by covalent bonds. First, we outline possibilities to improve the currently available approximations applied for the kinetic-energy component of the embedding potential. Second, we show how a generalized three-partition FDE scheme can be employed to circumvent the problems in the approximate kinetic-energy functionals by introducing capping groups, thus allowing for a subsystem DFT treatment of proteins.