Apart from having defects, crystals are not infinite either. They are terminated by surfaces. How to handle that with a DFT code with periodic boundary conditions? And can surface-specific properties be calculated that way?
This is a cif file for a minimalistic supercell for a bcc-V (001) surface: 3 atomic layers of vanadium, separated by 2 lattice parameters of vacuum. It was built based on the PBE-optimized bulk unit cell. Use your favourite DFT code to do a static calculation for this supercell (Quantum Espresso settings: ecutwfc=30 Ry, ecutrho=240 Ry, k-mesh=15x15x5 with this pseudopotential, this will take you about 20 minutes). Inspect the forces on the surface layer: does the surface layer want to move towards the vacuum or in the opposite direction? Determine the surface energy for this surface, without optimizing the positions of the layers (to save time). In case you would optimize the position of the surface layers, would the surface energy increase or decrease? Describe the steps you’d have to take to optimize the position of the layers (it is optional to actually perform this optimization).
If your DFT code allows to determine the Coulomb potential in the middle of the vacuum of a slab, then calculate the work function for this surface and compare it with experimental values (hands-on instructions are in the part with the exercices).
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