Fig. 3. Van der Waals corrected DFT calculations of the adsorption site for H${}_2$O on Bi${}_2$Te${}_3$(111).

The Bi and Te atoms are illustrated as purple and brown spheres, respectively. The energetically most favourable adsorption site is on top of the second layer Bi atom (a), followed by adsorption on top of the third layer Te atom (b) with a difference of 21 meV. Adsorption on top of the first layer Te atom (c) is the least favourable adsorption geometry, being 100 meV higher in energy than the absolute minimum. The optimised configuration of the water molecules for a $\left(2\times 2\right)$ supercell in d is essentially the same as for the higher coverage in a.