  Similarity transformed equation-of-motion coupled cluster theory (STEOM-CC)
is an alternative approach to equation-of-motion coupled cluster theory for
excited states (EOMEE-CC) which uses a second similarity transformation of the
Hamiltonian, followed by diagonalization in a small (CI singles-like)
excitation space, even when single and double excitations are included in the
transformation. In addition to vertical excitation energies, transition moments
measure the strength of the interactions between states determining absorption,
emission, and other processes. In STEOM-CCSD, transition moments are calculated
in a straight-forward manner as biorthogonal expectation values using both the
left- and right-hand solutions, with the main difference from EOMEE-CC being
the inclusion of the transformation operator. We recently developed an
extension of STEOM-CCSD to core excitations, CVS-STEOM-CCSD+cT, which includes
triple excitations and the well-known core-valence separation for the core
ionization potential calculations. In this work, we derived transition moments
for core-excited states with core triple excitations, including both
ground-to-core-excited and valence-to-core-excited transitions. The improvement
of the computed transition moments of the CVS-STEOM-CCSD+cT method is compared
to standard CVS-STEOMEE-CCSD and CVS-EOMEE-CCSD for our previously published
small molecule benchmark set.
