Surface slab geometries: clarify vacuum=None as a possibility for building electrodes

[tfrederiksen]

For building an electrode geometry, consistent with a given slab geometry, it may be convenient to use vacuum=None to get a crystal structure with the correct orientation, e.g.,

elec = sisl.geom.fcc_slab(alat, atoms, 111, layers=3, vacuum=None)
device = sisl.geom.fcc_slab(alat, atoms, 111, layers=9, vacuum=20) * (3, 3, 1)
device = device.sort(axis=[2, 1, 0])

I tried to make this possibility more clear in the documentation.

[codecov]

Codecov Report

Merging #514 (d06ec07) into main (ecd42bd) will increase coverage by 0.00%.
The diff coverage is 91.26%.

@@           Coverage Diff            @@
##             main     #514    +/-   ##
========================================
  Coverage   87.16%   87.17%            
========================================
  Files         353      353            
  Lines       45277    45391   +114     
========================================
+ Hits        39466    39569   +103     
- Misses       5811     5822    +11     

Impacted Files	Coverage Δ
sisl/geom/surfaces.py	96.64% <ø> (ø)
sisl/io/tbtrans/delta.py	92.73% <89.36%> (-1.11%)	⬇️
sisl/io/tbtrans/tests/test_delta.py	99.17% <96.87%> (-0.83%)	⬇️
sisl/geom/nanotube.py	94.79% <0.00%> (-1.05%)	⬇️

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[zerothi]

Great, thanks!