Massive Assessment\nof the Geometries of Atmospheric\nMolecular Clusters

Atmospheric molecular clusters are important for the\nformation\nof new aerosol particles in the air. However, current experimental\ntechniques are not able to yield direct insight into the cluster geometries.\nThis implies that to date there is limited information about how accurately\nthe applied computational methods depict the cluster structures. Here\nwe massively benchmark the molecular geometries of atmospheric molecular\nclusters. We initially assessed how well different DF-MP2 approaches\nreproduce the geometries of 45 dimer clusters obtained at a high DF-CCSD(T)-F12b/cc-pVDZ-F12\nlevel of theory. Based on the results, we find that the DF-MP2/aug-cc-pVQZ\nlevel of theory best resembles the DF-CCSD(T)-F12b/cc-pVDZ-F12 reference\nlevel. We subsequently optimized 1283 acid–base cluster structures\n(up to tetramers) at the DF-MP2/aug-cc-pVQZ level of theory and assessed\nhow more approximate methods reproduce the geometries. Out of the\ntested semiempirical methods, we find that the newly parametrized\natmospheric molecular cluster extended tight binding method (AMC-xTB)\nis most reliable for locating the correct lowest energy configuration\nand yields the lowest root mean square deviation (RMSD) compared to\nthe reference level. In addition, we find that the DFT-3c methods\nshow similar performance as the usually employed ωB97X-D/6-31++G(d,p)\nlevel of theory at a potentially reduced computational cost. This\nsuggests that these methods could prove to be valuable for large-scale\nscreening of cluster structures in the future.