Comparison of adamantane and fluoriteNiSi2

It has recently been proposed that surface-stabilized layers of adamantane ${\mathrm{NiSi}}_2$ as thick as 180 Å may exist on Si(100). We calculate the adamantane structure to have 1.54 eV less cohesive energy than the fluorite and conclude such thick layers cannot exist. Comparisons of the energy bands, projected densities of states, and charge densities give some insight into why fluorite is the preferred structure.