where n α and n β are the occupancies of the corresponding α, β frontier natural orbitals of the triplet state. For spin-pure open-shell states with zero spin projection, n α = n β . Figure shows (n αn β ) 2 versus S 2 , both computed for the EOM-SF-CCSD wave-functions of CH 2 as a function of the FNO threshold. A close correlation between the two quantities confirms that the above model of state mixing indeed captures the essense of the problem.