Assessment of Density Functional Theory in Predicting Structures and Free Energies of Reaction of Atmospheric Prenucleation Clusters

Abstract

This work assesses different computational strategies for predicting structures and Gibb’s free energies of reaction of atmospheric prenucleation clusters. The performance of 22 Density Functional Theory functionals in predicting equilibrium structures of molecules and water prenucleation clusters of atmospheric relevance is evaluated against experimental data using a test set of eight molecules and prenucleation clusters: SO₂, H₂SO₄, CO₂·H₂O, CS₂·H₂O, OCS·H₂O, SO₂·H₂O, SO₃·H₂O, and H₂SO₄·H₂O. Furthermore, the functionals are tested and compared for their ability to predict the free energy of reaction for the formation of five benchmark atmospheric prenucleation clusters: H₂SO₄·H₂O, H₂SO₄·(H₂O)₂, H₂SO₄·NH₃, HSO₄^–·H₂O, and HSO₄^–·(H₂O)₂. The performance is evaluated against experimental data, coupled cluster, and complete basis set extrapolation procedure methods. Our investigation shows that the utilization of the M06-2X functional with the 6-311++G­(3df,3pd) basis set represents an improved approach compared to the conventionally used PW91 functional, yielding mean absolute errors of 0.48 kcal/mol and maximum errors of 0.67 kcal/mol compared to experimental results