[DFTB-Plus-User] Transport Calculation Issues with Poisson Box and Poisson gate

[Pierre.Cazade]

Dear Alessandro,

Thank you for your message. Indeed, I maintained the periodicity along x- and y- (my system is defined as F in the gen file). So, that must be the source of the error.

I will send you a direct message as you suggested to discuss this further.

All the best,
Pierre
________________________________
From: DFTB-Plus-User <dftb-plus-user-bounces at mailman.zfn.uni-bremen.de> on behalf of ALESSANDRO PECCHIA <alessandro.pecchia at cnr.it>
Sent: Tuesday 4 February 2025 13:03
To: User list for DFTB+ related questions <dftb-plus-user at mailman.zfn.uni-bremen.de>
Subject: [DFTB-Plus-User] R: Transport Calculation Issues with Poisson Box and Poisson gate


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Dear Pierre,



I think there might be a problem if you have a periodic system along y and ask for a planar gate, which is also placed along y.

Yet, it works with periodic systems such as a graphene or MoS2, where there is a dummy periodicity (sufficiently large) along y.

A way forward is to check your structure myself. There might be some unexpected ‘problem’ because the gates were not tested too extensively in the dftb+ code.

According to the error message it seems the Poisson solver does not converge or the operator turns out singular.

Indeed (from MUDPACK documentation):

ERROR

c     = -3  if the continuous elliptic pde is singular.  this means the

c          boundary conditions are periodic or pure derivative at all

c          boundaries and ce(x,y) = 0.0 for all x,y.  a solution is still

c          attempted but convergence may not occur due to ill-conditioning

c          of the linear system coming from the discretization.  the

c          ierror = -3 flag overrides the ierror=-4 and ierror=-5 nonfatal

c          flags.



It is not clear why it is singular since the gate is itself fixing a potential. Do you use localBC ?

You can send me an email to alessandro.pecchia at cnr.it



Regards,



Alessandro Pecchia



Da: DFTB-Plus-User <dftb-plus-user-bounces at mailman.zfn.uni-bremen.de> Per conto di Pierre.Cazade
Inviato: martedì 4 febbraio 2025 12:28
A: dftb-plus-user at mailman.zfn.uni-bremen.de
Oggetto: [DFTB-Plus-User] Transport Calculation Issues with Poisson Box and Poisson gate



Dear users,



I have issues when performing transport calculations on a periodic 2-lead system using a parallel virtual gate in the Poisson calculation. I systematically get the following error:



ERROR!
-> Poisson solver error n=-3



I have tried everything I can think of but to no avail. Is there a systematic approach (gate size or distance to the junction) to make these calculations work? Please note that my system is built so that the transport direction is along the z-axis, while the leads are periodic in the x- and y-directions.



I have a different issue when trying to do an SCC transport calculation on a non-periodic 3-lead system. The non-SCC works fine but the Poisson box fails to initialize when attempting a SCC calculation:



ERROR!
-> Failure during initialisation of the Poisson box



I have tried different box sizes from matching the exact size of the system to having a padding of 20 Angstrom on every side, again to no avail. Any suggestions?



Best regards,

Pierre
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