The results of the examination of the acid ionization of HCl in water via a combination of ab initio electronic structure calculations and Monte Carlo simulations are described. The following key aspects are taken into account: the electronic structure change of the solute reaction system induced by the solvent polarization, the quantized nature of the proton nuclear motion, the solvent fluctuation and reorganization along with the solvent effects on the proton potential, and a Grotthuss mechanism of the proton transfer in water. The mechanism is found to involve stepwise transfers. The first is an almost activationless motion in a solvent coordinate, adiabatically followed by the quantum proton to produce a contact ion pair Cl--H3O+, which is stabilized by similar to 7 kcal/mol. The second is a motion in the solvent with a small activation barrier, as the adiabatic proton transfer produces a solvent-separated ion pair from the contact ion pair in an almost thermoneutral process. Motion of a neighbouring water molecule, accomodating a primary coordination number change of a proton-accepting water, is identified as a key in the reaction promoting solvent reorganization.
