High accuracy ab initio potential energy surface for the H₂O-H van der Waals dimer

A representation of the three-dimensional potential energy surface (PES) of the H₂O-H van der Waals dimer is presented. The H₂O molecule is treated as a rigid body held at its experimentally determined equilibrium geometry, with the OH bond length set to 1.809 650 34 a₀ and the HOH bond angle set to 1.824 044 93 radians. Ab initio calculations are carried out at the coupled-cluster single, double, and perturbative triple level, with scalar relativistic effects included using the second-order Douglas-Kroll-Hess approximation. The ab initio calculations employ the aug-cc-pVnZ-DK series of basis sets (n = D, T, Q), which are recontracted versions of the aug-cc-pVnZ basis sets that are appropriate for relativistic calculations. The counterpoise method is used to reduce the basis set superposition error; in addition, results obtained using the aug-cc-pVTZ-DK and aug-cc-pVQZ-DK basis sets were extrapolated to the complete basis set (CBS) limit. The PES is based on calculations carried out at 1054 symmetry-unique H₂O-H geometries for which the distance R between the H-atom and the H₂O center of mass ranges from R = 2.5-9.0 Å. The reproduction of the PES along the orientational degrees of freedom was performed using Lebedev quadrature and an expansion in spherical harmonics. The mean absolute error of the reproduced PES is <0.02 cm^-1 for R ≥ 3.0 Å and <0.21 cm^-1 for R between 2.5 and 3.0 Å. The global minimum for the CBS PES is a coplanar H₂O-H geometry, with R = 3.41 Å, in which the angle formed between the H₂O C₂ symmetry axis and the H-atom is 122.25°; the CBS binding energy for this geometry is 61.297 cm^-1. In addition, by utilizing the symmetry of the H₂O molecule, the spherical harmonic expansion was simplified with no loss in accuracy and a speedup of ∼1.8 was achieved. The reproduced PES can be used in future molecular dynamics simulations.