How many water molecules are needed to solvate one?

The comprehension at the molecular scale of the processes involved during solvation still remains a challenge in chemistry. Remarkably, the question concerning how many solvent molecules are necessary to solvate a solute one is still open. By exploring several water clusters of increasing size, we employ semiclassical spectroscopy [1-5] to determine on quantum dynamical grounds the minimal number of surrounding water molecules to make the central one display the same vibrational features of liquid water. We find out that the minimal structure eventually responsible of proper solvation is made of 21 water molecules, and that particular care must be reserved to the quantum description of the combination of the central monomer bending mode with network low-frequency librations.[6] The results obtained with the accurate ab initio potential are then compared with the popular Caldeira-Leggett one to rationalize whether a simplified model can qualitatively and quantitatively describe the solvated system behavior.[7] An ongoing study on how genetic algorithms[8] and adiabatically switched trajectories[9] can help to deconstruct the complex spectrum of the formic acid dimer will be also presented. [1] E. J. Heller, Acc. Chem. Res. 14, 368-375 (1981). [2] M. F. Herman and E. Kluk, Chem. Phys. 91, 27-34 (1984). [3] A. L. Kaledin and W. H. Miller, J. Chem. Phys. 119, 3078-3084 (2003). [4] M. Ceotto, S. Atahan, G. F. Tantardini and A. Aspuru-Guzik, J. Chem. Phys. 130, 234113 (2009). [5] M. Ceotto, G. Di Liberto and R. Conte, Phys. Rev. Lett. 119, 010401 (2017). [6] A. Rognoni, R. Conte and M. Ceotto, Chem. Sci. 12, 2060 (2021). [7] A. Rognoni, R. Conte and M. Ceotto, J. Chem. Phys. 154, 094106 (2021). [8] M. Gandolfi, A. Rognoni, C. Aieta, R. Conte and M Ceotto, J. Chem. Phys. 153, 204104 (2020). [9] R. Conte, L. Parma, C. Aieta, A. Rognoni and M. Ceotto, J. Chem. Phys. 151, 214107 (2019).