High-level ab initio and DFT calculations are employed to calculate the geometries of monosolvated Hg and Hg2+. In agreement with previous studies on M2+.H2O species, we calculate the equilibrium geometry of the Hg2+.H2O dicationic complex as having the Hg2+ interacting with the oxygen atom of water, but we find that the minimum energy geometry is nonplanar and attribute this to covalency. For Hg.H2O, in contrast to many previous studies on neutral M.H2O species, but in agreement with our previous studies, we find that the Hg atom prefers to be situated on the hydrogen end of the water molecule, in a C-s orientation. We rationalize this in terms of electron-electron repulsion. We calculate the energies of the lowest states of HgOH+ and conclude that the ground state is a bent, closed-shell (1)A' state, with a fair amount of covalency. CCSD(T) calculations employing very large basis sets, and employing the above geometries, allow us to calculate values for the interaction energies of Hg.H2O (213 cm(-1)) and Hg2+.H2O (90 kcal mol(-1)). In addition, the enthalpy of reaction for the process Hg2+ + H2O --> HgOH+ + H+ is calculated to be -40 kcal mol(-1) at the highest level of theory used herein. Finally, we conclude that the Hg2+.H2O complex should be observable, but that care in its preparation is required.