Coupled Cluster Free Energies for Atmospheric Molecular Clusters: Benchmark and Matching Experimental Free Energies

The initial formation of secondary aerosols, a large cause of uncertainty in modern radiative forcing modeling, can be simulated using quantum chemical methods. When based on quantum chemistry, the simulations have an exponential dependence on the free energy, requiring a high-accuracy description. In this study, we have computed harmonic frequencies and quasi-harmonic free energies for a set of 11 monomers and 27 dimers relevant for atmospheric molecular clusters at the DF-CCSD-(F12b)-(T*)/cc-pVDZ-F12 level of theory. The set is used to benchmark the M06-2X, PW91, ωB97X-D3BJ functional with the Jensen, Karlsruhe, and Pople style basis sets. The composite methods B97-3c, r²SCAN-3c, and ωB97X-3c are also tested. We find ωB97X-D3BJ/ma-def2-SVP to be an optimal choice, as it has low errors (mean absolute error of 0.13 kcal/mol) and few outliers in the thermal contribution. For calculations on larger clusters, B97-3c stands out. Using the ωB97X-D3BJ/ma-def2-SVP and B97-3c level of theories, we compute anharmonic frequencies using the VPT2 method and determine an anharmonic scaling factor of 0.961 and 0.953 for ωB97X-D3BJ/ma-def2-SVP and B97-3c frequencies, respectively. The scaling factor together with incorporating multiconformer entropy effects and a high-level single-point correction at the Normal LNO-CCSD-(T)/CBS-(aug-3,aug-4) are used to compare to experimentally determined free energies of the 11 hydrogen-bonded systems. We find that we obtain sub 1 kcal/mol errors when incorporating the scaling factor, the multiconformer entropy effects, and the single point correction.