Dear all!
In DFT supercells are used to study surfaces by introducing artificial vacuum. But some publications use supercells even though they study (infinite periodic) bulk band-structures. Is there a need to use a larger cell (considering PBC)?
I know that in mesoscopic models (molecular dynamics) there is a strong size dependence due to long-range forces, but does the same hold true for DFT (and if so, which potentials?)?
Details from practice: I use Quantum Espresso, (ionic) alloy semiconductors such as GaAs or PbS are of interest to me.
Every comment is greatly appreciated!
Yours,
Chris
Hi, Christoph.
If you are interested in bulk properties, the primitive cell should be enough (although you have to be careful sampling the reciprocal space!)
Of course, it depends on the property you want to study. Example: band gap and levels near the Fermi level -> Primitive cell + good (huge) k-sampling is perfect.
Fernando
In case of "normal" solid state calculation a supercell is not needed. As already mentioned, only the number of k-points have to adapted with respect to the system (i.e., fine k-sampling for metals). A supercell is used, for instance, to simulate partial site occupation, where with larger cell some of the site can be left unoccupied (of course, taking the symmetry properly into account).
It depends on what you're calculating. For 'normal' bulk calculations, a larger k-mesh is equivalent to a larger supercell. But it may be that you need a larger supercell for other reason: for example to discover whether symmetry-breaking structural or magnetic states can appear. Other possible reasons are to calculate the effects of defects, or the effects of a core-hole (Z+1 calculations for spectroscopy) where you want to make sure that the defect/core-hole doesn't interact with its image if the supercell is too small.
Dear all!
Thank you for your insightful answers. As I was already aware that defects/surfaces (i.e. not perfectly periodic "bulks") require supercells I am happy to learn from Alexandru about other examples where supercells are required!
Yours,
Chris
Supercell is needed if and when long range interaction (interactions exceeded 0.5 the cell dimension/s) need to be isolated, in order to model them.