N_BULK_LAYERS

N_BULK_LAYERS defines how many layers, counting from the bottom, form the repeat unit of the bulk structure. Layers are defined by the LAYER_CUTS parameter.

Note

The BULK_LIKE_BELOW parameter offers an easy way to detect the bulk repeat unit automatically, which will set both N_BULK_LAYERS and LAYER_CUTS. If BULK_LIKE_BELOW is defined and the bulk is detected, N_BULK_LAYERS will be updated in the PARAMETERS file during initialization.

Default: 1

Allowed values: 1, 2

Syntax:

N_BULK_LAYERS = 2

If N_BULK_LAYERS = 2, the bottom two layers will be repeated, alternating between the two. If N_BULK_LAYERS = 1, only the bottom layer will be repeated. See also Fig. 12 and this example for an illustration of the layers and the bulk repeat unit.

The layers defined as bulk should form a bulk unit cell, or a larger bulk repeat unit. The restriction to only one or two layers is given by TensErLEED. If your smallest bulk unit cell consists of more than two layers, use the LAYER_CUTS parameter to manually combine those layers to only one or two layers, and set N_BULK_LAYERS accordingly. An example for such a case is the ABAC stacking sequence of the Ni₃Ti (D0₂₄) structure. There, one should define the two bottommost layer pairs (AB and AC) as single layers by cutting at the appropriate heights (using a list and restricting cutting by interlayer distance to higher layers in LAYER_CUTS), and N_BULK_LAYERS = 2.

By default, as many layers above the bulk as are present within the bulk are used to determine the bulk repeat vector, see BULK_REPEAT. If not enough bulk-like layers are found, BULK_REPEAT will instead be defined as parallel to the POSCAR c vector, using only the z position of the bottommost non-bulk atom. See the page on the BULK_REPEAT parameter for more complicated cases.

Correct identification of the bulk layers and bulk repeat vectors can be checked by looking at the POSCAR_bulk and POSCAR_bulk_appended files, created during initialization.

See also: BULK_LIKE_BELOW, another way to specify which layers of the input file are used as bulk.