where Φ 0 is a reference determinant, which defines the many-body vacuum state and, consequently, the occupied and virtual orbital spaces , e T |Φ 0 is a CC wave function, and R is a general excitation operator . Explicit form of R depends on the level of correlation included in the model (e.g., it generates up to doubly excited configurations at the EOM-CCSD level) and on the type of the target states. For example, in EOM-CCSD for excitation energies , R is spin-and particle-conserving, i.e., of 1h1p and 2h2p types at the CCSD level, with an additional constraint that in each operator the number of α holes equals the number of α particles and the number of β holes equals the number of β particles. In EOM-CCSD for ionized (EOM-IP-CCSD) or electron-attached (EOM-EA-CCSD) states, R s are not particle conserving. In EOM-IP-CCSD, R is of 1h and 2h1p types and in EOM-EA-CCSD, R is of 1p and 2p1h types. In the spin-flip variant (EOM-SF-CCSD), operators R change the spin-projection by flipping the spin of an electron.