Based on density functional theory (DFT) within the generalized gradient approximation (GGA) we have calculated a three-dimensional (3D) potential energy surface (PES) for H-2+Pd(lll), depending on two hydrogen molecular degrees of freedom and one palladium surface degree of freedom. The PES is then used in 3D quantum mechanical wave packet calculations to investigate the effect of the surface motion on the direct subsurface absorption, We also compare the full 3D calculations to calculations where the surface motion is included through the sudden approximation. The calculations show a large downward shift of the onset energy for direct subsurface absorption upon inclusion of palladium surface motion (from 0.74 eV to 0.40 eV for H-2), The use of the sudden approximation works well at collision energies greater than 1.3 eV (for H-2), but lends to a significant underestimation of the direct subsurface absorption probability at lower energies, (C) 1997 American Institute of Physics.