where T is an excitation operator. Because Ĥ has the same spectrum as the bare Hamiltonian regardless of the choice of T , solving the EOM eigen problem in the full configurational space, i.e., when R includes all possible excitations, recovers the exact (full CI, FCI) limit. In practical calculations, these operators are truncated, most often to single and double excitations giving rise to EOM-CCSD ansatz, and the choice of the operator T becomes important.