(a) The center of mass coordinate system used for the description of the H2 molecule relative to the static Pt(111) surface. (b) The surface unit cell and the sites considered for the Pt(111) surface, and the relationship with the coordinate system chosen for H2 relative to Pt(111). The origin (X,Y,Z) = (0,0,0) of the center of mass coordinates is located in the surface plane at a top site. Polar and azimuthal angles θ and φ are chosen such that (θ = 90◦, φ = 0◦) corresponds to molecules parallel to the surface along the X (or equivalently U) direction.