Effects of spin contamination on estimating bond dissociation energies of polycyclic aromatic hydrocarbons

The objective of this work is to investigate the impact of spin contamination on the prediction of the enthalpies of formation of Polycyclic Aromatic Hydrocarbon (PAH) radicals and of the bond dissociation energies of their precursor molecule. These PAH radicals play a major role in the mass growth of soot precursors leading ultimately to the first soot particles. In this work, we highlights the errors due to spin contamination by comparing spin‐unrestricted open‐shell calculations (UHF, UMP2, and Quadratic CI singles and doubles [QCISD(T)]) with spin‐restricted open‐shell calculations (ROHF, ROMP2, and ROCCSD(T)). The results suggest that one should be very careful using any of the spin‐unrestricted methods (even QCISD (T)) unless the values are extremely close to the theoretical value. Following these observations, we propose a new set of best‐estimates for the enthalpies of formation of these critical PAH radicals using spin‐restricted open‐shell ROMP2 and RCCSD(T) calculations.