A thermodynamic view on the microsolvation of ions by rare gas: application to Li+ with argon

We present a thermodynamic perspective of microsolvation of ions by rare gas atoms, which is based on Parallel Tempering Monte Carlo (PTMC) simulations. This allows to establish a clear relationship between the structure of the solvation shells and the heat capacity (CV) as function of the number of individual solvent species. The dependence of CV on the temperature allows to identify internal structure rearrangements and the onset of partial or total melting of the clusters. As an application, we have employed the PTMC technique to study the thermodynamic properties of clusters resulting from the microsolvation of Li+ by argon atoms. Specifically, calculations have been carried out for the clusters Li+Arn (n=4-18, 33, 34, and 38) by applying two different potential energy surfaces (PESs): one includes only two-body interactions, while the other also incorporates three-body contributions. Whenever possible, we compare the present thermodynamics results with global optimization studies carried out previously (Prudente, Marques and Pereira, Phys. Chem. Chem. Phys., 2017, 19, 25707; Jesus et al., Int. J. Quantum Chem., 2019, 119, e25860). We conclude that the melting process arises for lower temperatures when the model PES accounts for three-body interactions. Additionally, we characterize the melting processes of the first and second solvation shells. For some specific clusters, structural rearrangements of the most external argon atoms are observed at very low temperatures.