  One-electron reduced density matrices (1RDMs) from equation-of-motion (EOM)
coupled-cluster with single and double excitations (CCSD) calculations are
analyzed to assess their N-representability ({\em i.e.}, whether they are
derivable from an physical N-electron state). We identify EOM-CCSD stationary
states whose 1RDMs violate either ensemble-state N-representability conditions
or pure-state conditions known as generalized Pauli constraints (GPCs). As
such, these 1RDMs do not correspond to any physical N-electron state.
Unphysical states are also encountered in the course of time-dependent EOM-CC
simulations; when an external field drives transitions between a pair of
stationary states with pure-state N-representable 1RDMs, the 1RDM of the
time-dependent state can violate ensemble-state conditions. These observations
point to potential challenges in interpreting the results of time-dependent
EOM-CCSD simulations.
