Communication: A new ab initio potential energy surface for HCl-H2O, diffusion Monte Carlo calculations of D0 and a delocalized zero-point wavefunction

We report a global, full-dimensional, ab initio potential energy surface describing the HCl-H2O dimer. The potential is constructed from a permutationally invariant fit, using Morse-like variables, to over 44 000 CCSD(T)-F12b/aug-cc-pVTZ energies. The surface describes the complex and dissociated monomers with a total RMS fitting error of 24 cm(-1). The normal modes of the minima, low-energy saddle point and separated monomers, the double minimum isomerization pathway and electronic dissociation energy are accurately described by the surface. Rigorous quantum mechanical diffusion Monte Carlo (DMC) calculations are performed to determine the zero-point energy and wavefunction of the complex and the separated fragments. The calculated zero-point energies together with a D-e value calculated from CCSD(T) with a complete basis set extrapolation gives a D-0 value of 1348 +/- 3 cm(-1), in good agreement with the recent experimentally reported value of 1334 +/- 10 cm(-1) [B. E. Casterline, A. K. Mollner, L. C. Ch'ng, and H. Reisler, J. Phys. Chem. A 114, 9774 (2010)]. Examination of the DMC wavefunction allows for confident characterization of the zero-point geometry to be dominant at the C-2v double-well saddle point and not the C-s global minimum. Additional support for the delocalized zero-point geometry is given by numerical solutions to the 1D Schrodinger equation along the imaginary-frequency out-of-plane bending mode, where the zero-point energy is calculated to be 52 cm(-1) above the isomerization barrier. The D-0 of the fully deuterated isotopologue is calculated to be 1476 +/- 3 cm(-1), which we hope will stand as a benchmark for future experimental work. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799231]