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Reading some Gaussian related documents (inputs, manuals, books), I found the possibility to setup periodic boundary conditions (PBC) in Gaussian.

As Gaussian is developed only for molecular systems (that are not periodic by nature), what does this mean? How is it useful?

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A periodic boundary condition creates a box around your molecule (or surface) and then treats the molecule leaving the box on the right side as entering the box from the left. This is particularly useful for surfaces as otherwise the atoms at the edges (but not on top where you'd be exposed to a solvent) would otherwise degrade into open space. This preserves the physicality of a surface extending to a much larger size than is computationally feasible.

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    $\begingroup$ And to piggyback off this, Gaussian is not only meant for molecular systems. Rather, most people happen to use it for molecular systems. You can use it for periodic systems with its PBCs -- it's just not particularly known for that. Of course, whether it is wise to use Gaussian for such a purpose is another question. $\endgroup$ – Andrew Rosen May 4 '20 at 18:25
  • $\begingroup$ @Andrew, the only version I saw for periodical system (that can calculate band structure for example) was from some research group that modified the Gaussian sources. As sold, I didn't find how to do that such of calculation. Do you have any reference about Gaussian used for periodical systems, aka crystals? $\endgroup$ – Camps May 4 '20 at 19:33
  • $\begingroup$ @Raz, what should be the difference in doing the calculation for some molecule with or without PBC? In case of MD, I understand why, but for a single molecule? $\endgroup$ – Camps May 4 '20 at 19:34
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    $\begingroup$ @I.Camps, To the best of my knowledge, Gaussian can't calculate a band structure without modification, but you can do a geometry optimization with PBCs. In terms of a material where you might do this, molecular solids immediately come to mind. I arbitrarily picked a paper from Google Scholar that shows PBC usage with Gaussian here. Besides, the fact that Gaussian allows for PBCs means you could (if your license allows) modify the source code to extend the capabilities further. $\endgroup$ – Andrew Rosen May 4 '20 at 21:18
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    $\begingroup$ I should be clear in that I'm not suggesting Gaussian be used for this purpose, and indeed there may not be many scenarios where you personally would find use for it. But your question was about what the meaning of PBCs are, and I feel that has been well-addressed here. There is nothing stopping you from modeling a bulk solid with PBCs in Gaussian. It's just... there are better options out there. $\endgroup$ – Andrew Rosen May 4 '20 at 21:20

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