  We determine the ground-state structure of a double vacancy in a hydrogen
monolayer on the Pd(111) surface. We represent the double vacancy as a triple
vacancy containing one additional hydrogen atom. The potential-energy surface
for a hydrogen atom moving in the triple vacancy is obtained by
density-functional theory, and the wave function of the fully quantum hydrogen
atom is obtained by solving the Schr\"odinger equation. We find that an H atom
in a divacancy defect experiences significant quantum effects, and that the
ground-state wave function is centered at the hcp site rather than the fcc site
normally occupied by H atoms on Pd(111). Our results agree well with scanning
tunneling microscopy images.

Materials Science