Theoretical study of the formation of mercury (Hg2+) complexes in solution using an explicit solvation shell in implicit solvent calculations. 
The structures and harmonic vibrational frequencies of water clusters (H2O)n, n = 1-10, have been computed using the M06-L/, B3LYP/, and CAM-BLYP/cc-pVTZ levels of theories.  On the basis of the literature and our results, we use three hexamer structures of the water molecules to calculate an estimated "experimental" average solvation free energy of [Hg(H2O)6](2+).  Aqueous formation constants (log K) for Hg(2+) complexes, [Hg(L)m(H2O)n](2-mq), L = Cl(-), HO(-), HS(-), and S(2-), are calculated using a combination of experimental (solvation free energies of ligands and Hg(2+)) and calculated gas- and liquid-phase free energies.  A combined approach has been used that involves attaching n explicit water molecules to the Hg(2+) complexes such that the first coordination sphere is complete, then surrounding the resulting (Hg(2+)-Lm)-(OH2)n cluster by a dielectric continuum, and using suitable thermodynamic cycles.  This procedure significantly improves the agreement between the calculated log K values and experiment.  Thus, for some neutral and anionic Hg(II) complexes, particularly Hg(II) metal ion surrounded with homo- or heteroatoms, augmenting implicit solvent calculations with sufficient explicit water molecules to complete the first coordination sphere is required-and adequate-to account for strong short-range hydrogen bonding interactions between the anion and the solvent.  Calculated values for formation constants of Hg(2+) complexes with S(2-) and SH(-) are proposed.  Experimental measurements of these log K values have been lacking or controversial.