where ω 1 and ω 2 are the energies of the two photons absorbed such that ω 1 + ω 2 = E f -E g , Ψs are the zero-order wave functions, and μx (μ y ) is the dipole moment operator along the Cartesian coordinate x (y) ∈ {x, y, z}. Within the EOM-EE-CCSD expectation-value approach, Eqs. ( ) and ( ) are parameterized by EOM-EE-CCSD wave functions and energies by replacing