LAYER_CUTS

LAYER_CUTS determines how the slab provided in the POSCAR file is divided in layers. Full dynamic multiple scattering is calculated inside each layer. The electron beams are then propagated as attenuated plane waves in between adjacent layers.

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. However, be careful combining BULK_LIKE_BELOW with LAYER_CUTS because the former will overwrite the latter during initialization producing a fixed list of cut positions. Particular care should be taken in adapting dc(float) and dz(float) cut types after initialization.

Allowed values: list of floats (0 < x < 1), dz(float), or dc(float)

Default: dz(1.2)

Syntax:

LAYER_CUTS = 0.09 0.17 0.22 0.39 0.65    ! explicit list of c values
LAYER_CUTS = dz(1.0)                     ! cut wherever distance along z is greater than 1.0 Å
LAYER_CUTS = dc(1.2)                     ! cut wherever distance along c is greater than 1.2 Å
LAYER_CUTS = 0.3 < dz(1.2) < 0.45 0.52   ! cut at c = 0.3, c = 0.45 and c = 0.52, and wherever distance along z is greater than 1.2 Å in range [0.3, 0.45]
LAYER_CUTS = 0.15 0.3 < dz(1.2)          ! cut at c = 0.15, c = 0.3, and wherever distance along z is greater than 1.2 Å above c = 0.3

  • The list input explicitly contains the positions at which the layers should be cut (order does not matter). These positions are expressed as a fraction of the height of the cell as defined by the c vector in the POSCAR file. The list should not contain the edges of the cell (i.e., 0 and 1). The topmost layer goes from c = the largest value in the list to c = 1.0.

  • dc(<float>) will automatically generate a list of cut positions, such that the distance along the c direction between the highest atom of each layer and the lowest atom of the layer above is larger than float (in Ångström).

  • dz(<float>) works similarly to dc(<float>), but the distances are expressed along the z cartesian coordinate (in Ångström), i.e., perpendicular to the plane of the a and b unit vectors.

  • Both dz and dc can be limited in scope by combining them with < or > characters, to specify that automatic cuts should be applied only above or below the values to the left and/or right. An example where this may be useful would be to specify the cuts in the bulk (which must not contain more than two layers) explicitly, but still get automatic cuts above.

Note

  • When specifying a list or limiting the c range of dz, take special care when a reference calculation is run from a previous optimized slab. Atoms may have moved substantially during the structural optimization (search), and can end up in the wrong layer if the LAYER_CUTS is given as a list of cut positions.

  • The scattering calculations in the underlying TensErLEED package can become unstable the distances between layers are less than than approximately 1.0 Å. Thus, the program will output a warning in case the smallest interlayer distance resulting from LAYER_CUTS is smaller than 1.0 Å. To avoid this issue, it is advisable to use the dz(float) version of LAYER_CUTS, with float \(\geq\) 1.

  • If your system has only interlayer distances significantly smaller than 1.0 Å, the only viable option is treating the whole slab as a single layer. Notice that one should be very careful in handling this case, and the computing time will be high. In order to treat this, the POSCAR input should contain a very thick slab (at least three times the largest inelastic mean free path for the energy range used) such that all beams have negligible amplitude when ‘exiting’ the bottom surface. This should be constructed by manually adding layers with the bulk structure at the bottom of the slab. In addition, one should introduce at least one ‘fake’ bulk layer with an appropriate cut position (use the list version of LAYER_CUTS), since the code requires the presence of a ‘bulk’ section of the slab. If the thick slab is thick enough to prevent transmission of electrons towards this fake bulk layer, its presence should not affect the results. It is advisable to iteratively test the minimum thickness of the slab: run a reference calculation with a certain number N of true bulk layers + one fake bulk repeat; then a second reference calculation with N+1 true bulk layers + the fake bulk repeat. Then run an \(R\)-factor calculation between the theoretical beams of the two reference calculations to confirm that the difference in \(R\) factor is small enough for your purposes.