Interpretation of TDA-TDDFRT auxiliary states

Determine whether the constitution, structure, or derivation of the TDA-TDDFRT central response problem can unequivocally establish an interpretation of the auxiliary state ψ_x, including whether it represents an excited-state wave function, an auxiliary state for constructing reduced quantities, or has no such interpretation.

Background

In the Tamm–Dancoff limit of TDDFRT, the paper considers the state ψ_x obtained by applying the excitation component of the response operator to the Kohn–Sham reference state. Unlike the TDHF/CIS setting, the authors state that there is no universal consensus about whether these objects should be interpreted as approximations to ground- and excited-state wave functions.

This unresolved interpretational issue affects whether conclusions established for TDHF can be transferred to TDDFRT, particularly concerning the physical acceptability and uniqueness of candidate one-body reduced difference-density matrices.