Single-determinant methods for electronic excited states

Traditionally, single-determinant methods are used to model ground electronic states of molecules, and multi-determinant methods are used to model excited electronic states.  However, there is no fundamental justification for this dichotomy and, indeed, it may be criticized as inconsistent.  We have therefore embarked on a systematic investigation of the extent to which single-determinant methods (such as Hartree-Fock theory and DFT models) may be useful for the treatment of ground states.