Hydration of Atmospheric Molecular Clusters III: Procedure for Efficient Free Energy Surface Exploration of Large Hydrated Clusters

Sampling the shallow free energy
surface of hydrated atmospheric
molecular clusters is a significant challenge. Using computational
methods, we present an efficient approach to obtain minimum free energy
structures for large hydrated clusters of atmospheric relevance. We
study clusters consisting of two to four sulfuric acid (sa) molecules
and hydrate them with up to five water (w) molecules. The structures
of the “dry” clusters are obtained using the ABCluster
program to yield a large pool of low-lying conformer minima with respect
to free energy. The conformers (up to ten) lowest in free energy are
then hydrated using our recently developed systematic hydrate sampling
technique. Using this approach, we identify a total of 1145 unique
(sa)2–4(w)1–5 cluster structures.
The cluster geometries and thermochemical parameters are calculated
at the ωB97X-D/6-31++G­(d,p) level of theory, at 298.15 K and
1 atm. The single-point energy of the most stable clusters is calculated
using a high-level DLPNO-CCSD­(T0)/aug-cc-pVTZ method. Using
the thermochemical data, we calculate the equilibrium hydrate distribution
of the clusters under atmospheric conditions and find that the larger
(sa)3 and (sa)4 clusters are significantly more
hydrated than the smaller (sa)2 cluster or the sulfuric
acid (sa)1 molecule. These findings indicate that more
than five water molecules might be required to fully saturate the
sulfuric acid clusters with water under atmospheric conditions. The
presented methodology gives modelers a tool to take the effect of
water explicitly into account in atmospheric particle formation models
based on quantum chemistry.