kgrid denotes the k-points sampling of the Brillouin Zone.
How to set?¶
In the "Important Settings" tab of the Subworkflow Editor interface lie the choice for the size and density of the grid of reciprocal k-points employed for sampling the Brillouin Zone of the electronic structure of the crystal under investigation.
The size of the grid of k-points is typically defined in terms of its dimensions in the three-dimensional reciprocal space (for example 10 x 10 x 10), in conjunction with the distance by which the grid is shifted in reciprocal space in order to cover a wider volume of the irreducible (by symmetry) part of Brillouin Zone. Both of these settings can be entered in their respective entries, "dimensions" and "shifts", under the "kgrid" section.
Together with a sufficiently high plane-wave cutoff for the electronic wavefunction explained in this page, a sufficiently large size for this sampling grid is of paramount importance in establishing the overall accuracy of the corresponding calculation. For instructions on how to add a preliminary convergence calculation add-on to the current workflow, see this page.
Alternatively, the option "KPPRA" can be selected in the above procedure through the right-hand side "preferKPPRA" checkbox. It defines the density of the grid of k-points in terms of k-points per reciprocal atom. If the unit cell contains 2 atoms, then a 10 x 10 x 10 grid leads to KPPRA of 2000. The default minimum value for this KPPRA setting is indicated immediately below the "kgrid" header label.
Other types of reciprocal space grids¶
Similar considerations as the above-mentioned elements apply to other types of unit-specific settings, such as the qgrid settings for defining reciprocal grids of q-points for performing phonon calculations, and the associated igrid settings for the grids obtained through performing the transformation to and from the reciprocal and real space and subsequent interpolation (hence the "i"). The reader is referred to Ref. 1 for an example theoretical explanation of each of these concepts.