Embedding theory for excited states with inclusion of self-consistent environment screening

We present a general embedding theory of electronic excitations of a relatively small, localized system in contact with an extended, chemically complex environment. We demonstrate how to include the screening response of the environment into highly accurate electronic structure calculation of the localized system by means of an effective interaction between the electrons, which contains only screening processes occurring in the environment. For the common case of a localized system which constitutes an inhomogeneity in an otherwise homogeneous system, such as a defect in a crystal, we show how matrix elements of the environment-screened interaction can be calculated from density-functional calculations of the homogeneous system only. We apply our embedding theory to the calculation of excitation energies in crystalline ethylene.