An accurate H2-H2 interaction potential from first principles

We have calculated the potential energy surface extrapolated to the complete basis set limit using coupled-cluster theory with singles, doubles, and perturbational triples excitations [CCSD(T)] for the rigid monomer model of (H-2)(2). There is significant anisotropy among the 37 unique angular configurations selected to represent the surface. A four term spherical harmonics expansion model was chosen to fit the surface. The calculated potential energy surface reproduces the quadrupole moment to within 0.58% and the experimental well depth to within 1%. The second virial coefficient has been computed from the fitted potential energy surface. The usual semiclassical treatment of quantum mechanical effects on the second virial coefficient was applied in the temperature range of 100-500 K. We have developed a new technique for computing the quantum second virial coefficient by combining Feynman's path integral formalism and Monte Carlo integration. The calculated virial coefficient compares very well with published experimental measurements. Integral elastic cross sections were calculated for the scattering of para-H-2/para-H-2 by use of the close-coupling method. The interaction potential model from this work is able to reproduce the experimental cross sections in the relative kinetic velocity range of 900-2300 m/s. (C) 2000 American Institute of Physics. [S0021-9606(00)30410-X].