Embedded equation-of-motion coupled-cluster theory for electronic excitation, ionization, and electron attachment

The projection-based quantum embedding method introduced by Manby et al. [J. Chem. Theory Comput. 8, 2564 (2012)] and extended to equation-of-motion coupledcluster singles and doubles (EOM-CCSD) theory by Bennie et al. [J. Chem. Phys. Lett. 8, 5559 (2017)] is applied to small molecules microsolvated by a varying number of water molecules. Using different variants of EOM-CCSD embedded in density functional theory, we investigate electronically excited states of valence, Rydberg, and charge-transfer character, valenceand core-ionized states, as well as bound and temporary radical anions. The latter states, which are unstable towards electron loss, are treated by means of a complex-absorbing potential. Besides transition energies, we also present Dyson orbitals and natural transition orbitals for embedded EOM-CCSD. We find that embedded EOM-CCSD reproduces results from full EOM-CCSD calculations very accurately if the lower-level fragment comprises only a single water molecule, whereas the performance deteriorates if more molecules are included in the environment. Our results illustrate that ionization potentials are a lot less sensitive towards technical details of the embedding procedure than excitation energies and especially electron attachment energies. No major difficulties are associated with combining projection-based embedding with non-Hermitian quantum chemistry for electronic resonances.