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I have noticed some difficulties in converging Transiesta calculations when including empty orbitals in the basis set, particularly when a non-zero bias is applied. In particular, I have tried to simply converge graphene structure and I could easily converge the calculation by excluding the empty orbitals. Including them, I could still converge zero-bias case but not the non-zero cases. Since I used quite long tails, I extended the divide region to approx. 6/7 layers. I understand that in the case of graphene, the energy window of interest is well described even by excluding these orbitals. However, I will include some atoms in which I included empty orbitals to properly describe their pristine band structure (in particular, B and N and I reproduced hBN band structure). My question is: is it advised not to include empty orbitals in transiesta calculations?

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    $\begingroup$ TBtrans is a 1 time execution code, there is no self-consistency in TBtrans, so what do you mean by "converging"? $\endgroup$
    – nickpapior
    Nov 25, 2022 at 21:02
  • $\begingroup$ I'm sorry, I meant the transiesta run before this. Again, I managed to converge it but it seems very slow. I imagined that generally speaking. adding more orbitals could make the calculation slower (and the convergence?) but I was winding if there is anything else I was missing. $\endgroup$
    – Laura
    Nov 27, 2022 at 11:08
  • $\begingroup$ @nickpapior this is the last day of the bounty, do you think you can help so that the $50 bounty doesn't go to waste? $\endgroup$ Dec 7, 2022 at 12:01

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There is per see nothing to prohibit the use of empty orbitals in TranSiesta calculations. In fact, in some cases they are very much required (vacuum spaces between STM tip and sample for instance).

However, there are a few details that require extra care.

  1. Adding empty orbitals in the scattering region, but not in the electrodes, may require an extended region of electrode layers between the actual electrode and the device region.
  2. The placement of empty orbitals can greatly affect the self-consistency depending on their relative position and how close they are to other orbitals.

So the answer is no to your question. But extra care should be taken when using empty orbitals in the device region and not in the electrode region.

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  • $\begingroup$ Thanks Nick. Can I ask you for more information about the cases in which they might be required? I have vacuum in some cases. WHat is their role in this case? $\endgroup$
    – Laura
    Dec 7, 2022 at 14:01
  • $\begingroup$ I'll mark this as answered for now! $\endgroup$
    – Laura
    Dec 7, 2022 at 16:01
  • $\begingroup$ @Laura just a reminder that you have only 12 hours remaining to award the bounty (if you want to award it to an answer). Maybe fewer than 12 hours when you actually see this. $\endgroup$ Dec 8, 2022 at 3:04
  • $\begingroup$ @Laura, what does empty orbitals do in an LCAO calculation? If you can answer this question, then you know what it does in all situations! :) $\endgroup$
    – nickpapior
    Dec 8, 2022 at 11:42

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