• EOM-EE-CCSD. . The EOM-EE excitation operator preserves the number of electrons and generates electronically excited states . For the closed-shell references, the excited states are naturally spin-adapted, forming singlet or triplet excitations. EOM-EE-CCSD can also be applied to open-shell references although in this the wavefunctions might be slightly or severely spin-contaminated , depending on the type of an excitation. This method is suitable for AB and to pAB, with the caveats discussed below.