Quantum Monte Carlo calculations of the dissociation energy of the water dimer

We report diffusion quantum Monte Carlo (DMC) calculations of the equilibrium dissociation energy D-e of the water dimer. The dissociation energy measured experimentally, D-0, can be estimated from D-e by adding a correction for vibrational effects. Using the measured dissociation energy and the modern value of the vibrational energy Mas , [J. Chem. Phys. 113, 6687 (2000)] leads to D-e=5.00 +/- 0.7 kcal mol(-1), although the result Curtiss , [J. Chem. Phys. 71, 2703 (1979)] D-e=5.44 +/- 0.7 kcal mol(-1), which uses an earlier estimate of the vibrational energy, has been widely quoted. High-level coupled cluster calculations Klopper , [Phys. Chem. Chem. Phys. 2, 2227 (2000)] have yielded D-e=5.02 +/- 0.05 kcal mol(-1). In an attempt to shed new light on this old problem, we have performed all-electron DMC calculations on the water monomer and dimer using Slater-Jastrow wave functions with both Hartree-Fock approximation (HF) and B3LYP density functional theory single-particle orbitals. We obtain equilibrium dissociation energies for the dimer of 5.02 +/- 0.18 kcal mol(-1) (HF orbitals) and 5.21 +/- 0.18 kcal mol(-1) (B3LYP orbitals), in good agreement with the coupled cluster results. (c) 2006 American Institute of Physics.