We present results of (v=0, j=0) HD reacting on and scattering from Pt(111) at off-normal angles of incidence, treating all six molecular degrees of freedom quantum mechanically. The six-dimensional potential energy surface (PES) used was obtained from density functional theory, using the generalized gradient approximation and a slab representation of the metal surface. Diffraction and rotational excitation probabilities are compared with experiment for two incidence directions, at normal incidence energies between 0.05-0.16 eV and at a parallel translational energy of 55.5 meV. The computed ratio of specular reflection to nonspecular in-plane diffraction for HD+Pt(111) is lower than found experimentally, and lower for HD+Pt(111) than for H-2+Pt(111) for both incidence directions studied. The calculations also show that out-of-plane diffraction is much more efficient than in-plane diffraction, underlining that results from experiments that solely attempt to measure in-plane diffraction are not sufficient to show the absence of surface corrugation. Discrepancies in rotational excitation and diffraction probabilities between theory and experiment are discussed, as well as possible future improvements in the dynamical model and in the calculation of the PES. (C) 2003 American Institute of Physics.