Nitrogen and sulfur compounds in atmospheric aerosols: A new parametrization of polarized molecular orbital model chemistry and its validation against converged CCSD(T) calculations for large clusters

The parametrization of the polarized molecular orbital (PMO) method, which is a neglect-of-diatomic-differential-overlap (NDDO) semiempirical method that includes polarization functions on hydrogens, is extended to include the constituents that dominate the nucleation of atmospheric aerosols, including ammonia, sulfuric acid, and water. The parametrization and validation are based mainly on CCSD(T)/CBS results for atmospheric clusters composed of sulfuric acid, dimethylamine, and ammonia and on M06-2X exchange-correlation functional calculations for other constituents of the atmospheric aerosols. The resulting model, called PMO2a, is parametrized for molecules containing any type of H, C, or O, amino or ammonium N, and S atoms bonded to O. The new method gives greatly improved electric polarization compared to any other member of the family of NDDO methods. In addition, PMO2a is shown to outperform previous NDDO methods for atomization energies and atmospheric aerosol reaction energies; therefore, its use can be recommended for realistic simulations.