INTERMOLECULAR POTENTIAL OF H2O...H2 IN THE VANDERWAALS REGION - AN ABINITIO STUDY

The fourth-order Moller-Plesset perturbation theory is used to evaluate the intermolecular potential of the H2O...H-2 system with special emphasis on the van der Waals well region. When interacting with H2O, the H-2 molecule can act either as a proton donor or as a proton acceptor. In the minimum energy configuration (-197 cm-1), H-2 approaches the O atom collinearly with the C2 axis of H2O. In the secondary attractive region (-184 cm-1), H-2 forms a T-shaped structure with the O-H bond of H2O (the H-2 axis is perpendicular to the H2O plane). Other attractive areas of the potential are also examined. The origins of anisotropy of the interaction potential are studied by dissecting the interaction energy into its components-electrostatic, exchange repulsion, dispersion, deformation, etc. The potential energy surface is highly anisotropic, due largely to electrostatic interactions.