I understand that the radical character is quantified by the occupation number of the lowest unoccupied natural orbital obtained from UHF calculations or from CASSCF calculations. My issue is that I already have an organic diradical whose ground state is high-spin state. In this case how can I quantify the radical character ? Is there a way to know about the mixing percentage from higher spin states which in turn could give us information about the radical character of the molecule ?
Take a look at this is the article, which provides quantitative characterization of radical character: M. Head-Gordon. Characterizing unpaired electrons from the one-particle density matrix. Chem. Phys. Lett., Vol. 372, Issues 3–4, 2003, p. 508-511
Based on natural orbitals, the analysis in this paper is applicable to any correlated method - it needs only one-particle state density matrix.