Ionization Energies of Polycyclic Aromatic Hydrocarbons: Benchmarking Semiempirical, Tight Binding, and Composite Methods

We benchmark semiempirical (PM6, PM7, RM1), tight-binding (DFTB2, DFTB3, xTB-IPEA, g-xTB), and composite methods (HF-3C, B97-3C, PBEH-3C, B3LYP-3C, R2SCAN-3C, ωB97X-3C) for first, second, and double ionization energies of selected polycyclic aromatic hydrocarbons (PAHs). All calculations use the ΔSCF approach with zero-point energy corrections. No single method is optimal across all ionization types. For first ionization energies, xTB-IPEA is the most accurate approximate method, while ωB97X-3C outperforms all composite approaches. ωB97X-3C and DFTB3 show the most even performance across the three ionization categories. Comparison of B3LYP-3C with literature B3LYP results reveals that composite corrections negligibly affect ionization energies, which are dominated by delocalization error effectively mitigated by range-separated functionals. These results guide the selection of cost-effective methods for PAH ionization studies.