Siesta

How to fix: WARNING: Please increase the electrode size in the transport direction.

Asked by Tristan Zaborniak

In running the 4.0 version of Transiesta on the structure contained in the dropbox link below, the warning message appears:

 WARNING: Connections across 2 unit cells or more in the transport direction.
 WARNING: This is inadvisable.
 WARNING: Please increase the electrode size in the transport direction.
 WARNING: Will proceed without further notice.
 WARNING: Connections across 2 unit cells or more in the transport direction.
 WARNING: This is inadvisable.
 WARNING: Please increase the electrode size in the transport direction.
 WARNING: Will proceed without further notice.

The user manual reads that if this warning is shown, one should try increasing the electrode layers and using the TS.NumUsedAtoms options to limit the number of atoms in the surface Green's function calculation. Further, it goes on to say that this error is often thrown in the case of the ABC stacking of gold.

I have never run across this problem, even in using essentially the same electrodes as these (which are gold), and have only ever used the default TS.NumUsedAtoms options. As such, I am unsure how to take the above advice.

My question is thus: for this structure, can you outline a reasonable means of getting around this error, given the structure that I am working with? Thanks!

https://www.dropbox.com/sh/3cqabwjzzfwikbb/AACZ4mg21A1_nF3ReBOMi3NVa?dl=0

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Nick Papior
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Nick Papior (nickpapior) said : 		#1

Like the error message and what you write. You have to increase the size of the electrode along the semi-infinite direction.
The requirement is that electrode cells must only have couplings to its nearest neighbour cell.

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Tristan Zaborniak (tristanz) said : 		#2

Yes, but... after increasing the length of the electrode in this way, what is the importance of the TS.NumUsedAtoms option?

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Best Nick Papior (nickpapior) said : 		#3

It can be used to reduce the number of atoms *actually* used in the electrode. In particular if you have ABCDABCD stacking in your electrode, but you could fulfill the nearest neighbour coupling using only ABCDAB, then this flag can be used to specify how many atoms to use.

However, if in doubt. Don't use that flag. :)

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Tristan Zaborniak (tristanz) said : 		#4

Thanks Nick Papior, that solved my question.