Extrapolating Local Coupled Cluster Calculations toward CCSD(T)/CBS Binding Energies of Atmospheric Molecular Clusters

Aerosols are the largest source of uncertainty in modern global radiative forcing modeling. Atmospheric molecular clusters are important intermediates in atmospheric new particle formation (NPF). The evaporation rate of clusters can be calculated using quantum chemical methods, with an exponential dependence on the free energy. Hence, for simulating accurate NPF rates, high-accuracy calculations are needed. We have constructed a versatile benchmark set of 218 conformers of atmospheric molecular dimer clusters consisting of sulfuric acid (SA), formic acid (FA), nitric acid (NA), methanesulfonic acid (MSA), water (W), ammonia (AM), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylenediamine (EDA) molecules. Using this test set, we benchmark the local coupled cluster methods, DLPNO-CCSD-(T₀) and LNO-CCSD-(T), using different basis sets and locality settings, and test extrapolation procedures to the complete basis set (CBS), local approximation free (LAF), and complete PNO space (CPS) limits. The extrapolations are tested against the binding energies of high-level CCSD-(F12*)-(T+)/cc-pVTZ-F12 reference calculations. We find that the LNO-CCSD-(T) methods offer a better accuracy-to-cost ratio for atmospheric molecular clusters than the usually employed DLPNO-CCSD-(T₀) method. Furthermore, the CBS limit extrapolation using the aug-cc-pVTZ and aug-cc-pVQZ basis sets should be readily attainable for the LNO-CCSD-(T) method on the usually studied cluster sizes (4-8 monomers). Simulating the new particle formation rate of the (SA)_1-4(AM)_1-4 and (SA)_1-4(DMA)_1-4 systems using the Atmospheric Cluster Dynamics Code, we find an increased sensitivity to the locality settings for larger clusters, but the basis set error is still the most dominant. Hence, simulated cluster formation rates would also benefit from doing LAF extrapolation. Finally, we illustrate the calculations of LNO-CCSD-(T)/CBS binding energies of a large (SA)₁₅(TMA)₁₅ cluster (300 atoms). Hence, the application of LNO-CCSD-(T) allows for significantly more accurate binding energies of much larger clusters than previously possible.