title: Embedding theory for excited states.
abstract: Using the technique of Perdew and Levy [Phys. Rev. B 31, 6264 (1985)], it is shown that both the density function theory (DFT)-in-DFT and wave function theory (WFT)-in-DFT embedding approaches are formally correct in studying not only the ground state but also a subset of the excited states of the total system. Without further approximations, the DFT-in-DFT embedding approach results in a pair of coupled Euler-Lagrange equations. In contrast to DFT-in-DFT, the WFT-in-DFT approach is shown to ensure a systematic description of excited states if such states are mainly related to excitations within the embedded subsystem. Possible ways for the practical realization of the WFT-in-DFT approach for studying excited states are briefly discussed.
authors.affiliation: Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202-9024, USA.
authors.forename: Yuriy G
authors.initials: YG
authors.lastname: Khait
authors.forename: Mark R
authors.initials: MR
authors.lastname: Hoffmann