Comparison of microhydration methods: Protonated glycine as a working example

The determination of the structures and relative energies of microsolvated complexes remains an important challenge for theoretical chemistry because the number of possible aggregates rapidly increases when more solvent molecules are considered. Several approaches (chemical intuition, hierarchy of scheme, evolutionary trees,...) have been proposed previously to identify the most stable geometries. In the present paper, we compare both the structures and interaction energies computed with different computational protocols for the model case of microhydrated protonated glycine, a system for which experimental complexation enthalpies are available. We compare the results obtained, one the one hand, by a hierarchical approach starting with a AMOEBA search followed by DFT and MP2 refinements and, on the other hand, by a Darwinian tree that uses evolutionary logic and counterpoise corrected MP2 calculations. It is shown that the two approaches yield relatively similar conclusions, though some structures can only be identified by one of the two strategies. This paper therefore underlines the interest of applying more than one minima search tactic for pinpointing all possible microsolvated complexes.