Ab Initio Potential for HO → H + HO: A Step to a Many-Body Representation of the Hydrated Proton?

We report a new potential energy surface (PES) for hydronium that dissociates to H + HO. The PES is a permutationally invariant fit to a data set of nearly 100,000 electronic energies, of which most are CCSD(T)-F12/aug-cc-pVQZ, plus a small set of MRCI-aug-cc-pVTZ diabatic energies in the region where the CCSD(T) method fails. The long-range part of the PES is described accurately by a classical Coulomb interaction between the proton and HO using partial charges obtained from an accurate, full-dimensional dipole moment surface. A switching function connects the fitted PES to this long-range interaction.The fidelity of this global PES is determined by a combination of standard geometry and harmonic analyses at the minimum and inversion saddle point. In addition, VSCF/VCI calculations of the fundamentals and tunneling splittings are reported; all of these are within 3 cm or less of experimental values. A diffusion Monte Carlo calculation is also reported for the zero-point state. The PES is used in a two-body representation of the interaction of the proton with two water molecules, including a 2-body HO-HO interaction, and is shown to give a realistic description of the Zundel cation HO. This demonstrates that the PES may be usable as a component in a many-body potential describing the hydrated proton, especially for vibrational calculations of protonated water clusters.