Consequently, the dynamic correlation in the singlet and triplet states is very similar, leading to rather small errors in the OSFNO calculations of the singlet-triplet gaps not exceeding 18 cm -1 (Figure ). Encouraged by small errors introduced by OSFNO, we carried out the EOM-SF-CCSD calculation for PATFIA with cc-pVTZ (1,038 orbitals in total). Using the threshold 99% of total population corresponds to freezing of 429 virtual orbitals. The resulting exchange constant differs by less than 1 cm -1 from the cc-pVDZ result, thus validating the computational protocol used in Ref . To benchmark OSFNO beyond energy gaps, we computed spin-orbit coupling constants (SOCC) in the methylene series between the triplet and closed-shell EOM-SF states within mean-field spin-orbit approximation using the algorithm described in Ref. . In all cases, we observed that the errors in SOCC due to OSFNO are less than 2.5 cm -1 , which is comparable with the accuracy of the mean-field approximation.