ABINITIO STUDY OF THE POTENTIAL-ENERGY SURFACE OF CH4-H2O

The potential energy surface of CH4-H2O is calculated through the fourth-order Moller-Plesset perturbation theory. In an attempt to obtain basis-set saturated values of interaction energies the extended basis sets are augmented by bond functions which simulate the effects of high-symmetry polarization functions. The absolute minimum occurs for the configuration involving the C-H-O hydrogen-bond in which O-H points toward one of the faces of the CH4 tetrahedron. The equilibrium C-O separation is equal to 6.8 a0 which corresponds to the bond energy of 0.83 kcal/mol. Due to basis set unsaturation of the dispersion energy the bond energy may still be underestimated by about 0.05 kcal/mol. The secondary minimum involving the C-H-O hydrogen-bond is some 0.2 kcal/mol less stable, and the corresponding. C-O distance is longer by 0.6 a0. The anisotropy of the potential energy surface is analyzed via the perturbation theory of intermolecular forces. The binding in CH4-H2O is chiefly due to the dispersion energy which sets the general trend for the anisotropy of the surface. A more detailed examination, however, indicates that the anisotropy of the surface results from a complex interplay of various factors, including electrostatics, exchange repulsion, and to a lesser degree, the deformation effects. Analysis of various exchangeless perturbation approximations to the deformation effect indicates that the neglect of exchange component of deformation may lead to an incorrect description of the van der Waals region. The analytical potential for the CH4-H2O interaction is provided.