Supercells, slabs, and vacuum
Cell repetition in the viewer allows inspecting extended lattices without modifying the underlying unit cell. Slabs and low-dimensional structures are constructed by explicitly introducing vacuum along the desired non-periodic direction (typically c).
Quantum ESPRESSO always assumes periodic boundary conditions in 3D: for a slab or 2D material, maintain sufficient vacuum separation between periodic replicas, set a single k-point along the vacuum direction (e.g. N × M × 1), and configure appropriate dipole/electrostatic corrections.
Common systems
| System | Display repetition | Computational checks |
|---|---|---|
| Bulk crystal | 2 × 2 × 2 | Converge a three-dimensional k mesh; use the intended primitive or conventional cell. |
| Slab / 2D sheet in a-b plane | 2 × 2 × 1 | Converge perpendicular vacuum and in-plane k mesh; usually one k-point along c. Review dipole or 2D isolation treatment when appropriate. |
| Wire along c | 1 × 1 × 2 | Converge vacuum in both transverse directions and sampling along the wire. The built-in c-vacuum action alone does not prepare this geometry. |
| Molecule / cluster | 1 × 1 × 1 | Use a sufficiently large 3D box; typically Gamma-only sampling. Check charged-system and dipole treatment as needed. |
A reproducible slab check
- Orient the slab so its surface lies in the a-b plane.
- Add vacuum along c and inspect the actual cell and positions.
- Set cell repetition to 2 × 2 × 1 to inspect the in-plane extended surface without repeating along the vacuum direction.
- Set the k mesh to N by M by 1; choose N and M through convergence.
- Inspect generated or expert input for the appropriate electrostatic setup.
- Repeat with larger vacuum and denser in-plane sampling until the target quantity is stable.