Massive Assessment of the Geometries of Atmospheric Molecular Clusters

Atmospheric molecular clusters are important for the formation of new aerosol particles in the air. However, current experimental techniques are not able to yield direct insight into the cluster geometries. This implies that to date there is limited information about how accurately the applied computational methods depict the cluster structures. Here we massively benchmark the molecular geometries of atmospheric molecular clusters. We initially assess how well different DF-MP2 approaches reproduce the geometries of 45 dimer clusters obtained at a high DF-CCSD(T)-F12b/cc-pVDZ-F12 level of theory. Based on the results we find that the DF-MP2/aug-cc-pVQZ level of theory best resembles the DF-CCSD(T)-F12b/cc-pVDZ-F12 reference level. We subsequently optimize 1283 acid–base cluster structures (up to tetramers) at the DF-MP2/aug-cc- pVQZ level of theory and assess how more approximate methods reproduce the geometries. Out of the tested semi-empirical methods, we find that the newly parameterized atmospheric molecular cluster extended tight binding method (AMC-xTB) is most re- liable for locating the correct lowest energy configuration and yield the lowest RMSD compared to the reference level. In addition, we find that the DFT-3c methods show similar performance as the usually employed ωB97X-D/6-31++G(d,p) level of theory at a potentially reduced computational cost. This suggests that these methods could prove valuable for large-scale screenings of cluster structures in the future.