I am currently learning DFTB+ using the tutorials on the official website :

  1. Carbon Nanoribbons : https://dftbplus-recipes.readthedocs.io/en/latest/defect/carbon2d-elec.html#zigzag-nanoribbon

  2. Molecular Junctions : https://dftbplus-recipes.readthedocs.io/en/latest/transport/molecule-junction.html

How do I go about creating the geometry files for the systems used in these tutorials?

So far, I have been using Avogadro for creating xyz files but I haven't created "planar periodic" structures as illustrated in the tutorials.

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Since you know how to use Avogadro to create XYZ files, a very simple solution to your problem is to simply create the XYZ file then use an xyz2gen script to convert fron XYZ format to the GEN format you seek. This is an example of a script that converts from XYZ to DFTB+ format.

The beauty of the GEN format of DFTB+ is that with only a few lines, you can "generate" the entire structure based on the periodicity of the system, which is specified by Cartesian lattice vectors. An excellent exercise to help you learn how this works is given at the top of the first page you gave us in your question. Start at the origin and add atoms based on the vectors given: you'll find it quite cool how a 2D hexagonal lattice begins to emerge!


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