In o,m,p-benzynes series, the computed singlet-triplet energy gaps (-0.99, -0.51, -0.14 eV) reflect an increased diradical character. In calculations with the UHF orbitals we observe the largest error due to OSFNO in the singlet-triplet gap for the meta isomer, growing up to 200 cm -1 at 99% of the total unfrozen population (Figure , top right). The large errors for m-benzyne can be attributed to the significant spin-contamination of the reference determinant. To mitigate the effect of spin contamination, we explored using the ROHF orbitals, PBE orbitals, and OO-MP2 orbitals (computed with the RI approximation for the MP2 part). To investigate the applicability of the OSFNO algorithm beyond diradicals, we considered two isomers of the dehydro-meta-xylylene (DMX) triradicals , 2-and 5-DMX. The previous work has shown that the ground state of 2-DMX is quartet 4 B 2 and the ground state of 5-DMX is doublet 2 B 2 . The computed doublet-quartet energy gaps are: 0.45 and -0.53 eV for 2-DMX and 5-DMX, respectively. Figure shows the errors in doublet-quartet gaps in 2-and 5-DMX. As in the case of benzynes, the UHF reference determinant of DMX is significantly spin-contaminated: S 2 is 4.45 for 2-DMX and 4.29 for 5-DMX. The spin contamination of the reference determinant is eliminated almost entirely using the PBE orbitals, which also significantly reduces the errors of OSFNO for doublet-quartet gaps.