Reparameterization of PM6 Applied to Organic Diradical Molecules

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Semiempirical molecular orbital (SE-MO) methods offer a balance between accuracy and computational cost.
• Existing spin-unrestricted SE-MO methods like UPM6 can suffer from spin contamination.
• Accurate computation of open-shell species, such as organic diradicals, is crucial in chemistry.

Purpose of the Study:

• To develop a reparameterized PM6 (rPM6) method for improved computation of open-shell species.
• To enhance the accuracy and reduce spin contamination in semiempirical calculations.
• To provide a computationally efficient alternative to density functional theory (DFT) for studying diradicals.

Main Methods:

• Reparameterization of the PM6 method using a training set of 740 reference data points.
• Simultaneous optimization of parameters for hydrogen, carbon, nitrogen, and oxygen.
• Application of the spin-unrestricted rPM6 (UrPM6) method to organic diradicals and polycyclic aromatic hydrocarbons.

Main Results:

• The mean absolute error of rPM6 for ground-state properties was reduced from 16.1 to 14.1 kcal/mol on the GMTKN30 database.
• UrPM6 demonstrated substantial improvement over standard SE-MO methods (UAM1, UPM3, UPM6) for diradical and PAH calculations.
• UrPM6 calculations showed significantly reduced susceptibility to spin contamination compared to previous methods.

Conclusions:

• The UrPM6 method provides accurate geometric parameters and adiabatic singlet-triplet energy gaps for open-shell systems.
• UrPM6 achieves DFT-level accuracy (UB3LYP/UBHandHLYP) at a much lower computational cost.
• This method represents a significant advancement for the theoretical study of organic diradicals and related species.