with a S = 2 for singlet, and a S = 0 for triplet spin. The Kohn-Sham matrix contributions F and kernel contributions K depend on the chosen functional and comprise one-electron, Coulomb and exchange-correlation (XC) contributions. The projection operator onto the virtual orbital space Q ensures orthonormality of excitation amplitudes X N with the ground-state MO coefficients C. The global parameter a EX scales the amount of exact exchange and the two-electron repulsion integrals are defined in Mulliken notation.