Orientation dependence of the hydrogen molecule’s interaction with Rh(001)

Abstract

First-principles calculational results, that conflict with conventional wisdom, are reported for the orientation dependence of the energy of ${\mathrm{H}}_2$, a few bohrs above a Rh(001) surface. In geometric configurations near the saddle point where H-Rh bonds begin to form, the favored molecular axis orientation is along a 〈100〉 direction, corresponding to the attraction of each H atom to a twofold bridge site. This is true even though the final destinations of the H atoms are fourfold hollows, and even though the 〈100〉 orientation corresponds to relatively low symmetry, i.e., ${\mathit{C}}_{\mathit{s}}$ rather than ${\mathit{C}}_{2\mathit{v}}$.