Accelerated Diradical Character Assessment in Large Datasets of Polybenzenoid Hydrocarbons Using xTB Fractional Occupation

Polybenzenoid hydrocarbons (PBHs) have gar- nered significant attention in the field of or- ganic electronics due to their unique electronic properties. To facilitate the design and discov- ery of new functional organic materials based on these compounds, it is necessary to assess their diradical character. However, this usually requires expensive multireference calculations. In this study, we introduce a novel approach for rapidly identifying and quantifying open- shell character in PBHs using the fractional occupation number weighted electron density metric (NFOD) calculated with the semiempir- ical xTB method. We apply this approach to the entire chemical space of PBHs containing up to 10 rings, a total of over 19k molecules. Our findings reveal a strong correlation be- tween xTB-calculated NFOD and the more com- putationally expensive Yamaguchi y and DFT- calculated NFOD, validating the use of this effi- cient method for large-scale screening. Addi- tionally, we identify a linear relationship be- tween size and NFOD value and implement a size-dependent threshold for open-shell charac- ter, which significantly improves the accuracy of diradical identification across the chemical space of PBHs. This size-aware approach re- duces false positive identifications from 6.97% to 0.38% compared to using a single thresh- old value. Overall, this work demonstrates that xTB-calculated NFOD provides a rapid and cost- effective alternative for large-scale screening of open-shell character in PBHs.