  Many-body techniques based on the double unitary coupled cluster ansatz
(DUCC) can be used to downfold electronic Hamiltonians into low-dimensional
active spaces. It can be shown that the resulting dimensionality reduced
Hamiltonians are amenable for quantum computing. Recent studies performed for
several benchmark systems using quantum phase estimation (QPE) algorithms
demonstrated that these formulations can recover a significant portion of
ground-state dynamical correlation effects that stem from the electron
excitations outside of the active space. These results have also been confirmed
in studies of ground-state potential energy surfaces using quantum simulators.
In this letter, we study the effectiveness of the DUCC formalism in describing
excited states. We also emphasize the role of the QPE formalism and its
stochastic nature in discovering/identifying excited states or excited-state
processes in situations when the knowledge about the true configurational
structure of a sought after excited state is limited or postulated (due to the
specific physics driving excited-state processes of interest). In this context,
we can view the QPE algorithm as an engine for verifying various hypotheses for
excited-state processes and providing statistically meaningful results that
correspond to the electronic state(s) with the largest overlap with a
postulated configurational structure. We illustrate these ideas on examples of
strongly correlated molecular systems, characterized by small energy gaps and
high density of quasi-degenerate states around the Fermi level.
