# Converging k-points in Transiesta/TBTrans

Asked by Tristan Zaborniak on 2019-12-03

I believe that there are some gaps in my understanding of how to converge k-points in Transiesta/TBTrans (version 4.0). Below I've compiled what I've read/know, asking questions where unsure.

Converging k-points in Transiesta:

1. For the electrodes, I've read that one should use a high number of k-points in the transport direction (~100), and then converge the transverse direction k-points. Is this so? I would've thought that because the electrode calculation is a Siesta calculation, one would have to converge k-points in the transport direction as well.

2. After the k-points of the electrode are converged, the transverse-direction k-points of the device should be the same as for those of the converged electrode. Is this true? And, because Transiesta truncates the transport k-points to 1, do we need to worry about converging k-points for the device region at all? Or do we converge for the device region first, and then use the transverse-direction k-points in defining those of the electrode?

Converging k-points in TBTrans:

3. Here I've read that one must converge transverse-direction k-points by looking at the averaged transmission. This is done by overlaying transmissions calculated using different k-point samplings, and finding the density at which the superimposed curves fall on top of one another, yes?

And, finally, regarding the last column of the k-points block: is it a good rule of thumb to set each entry (offset) to 0.5? Are there cases where setting each entry to 0, or some other number, is preferable?

## Question information

Language:
English Edit question
Status:
Solved
For:
Siesta Edit question
Assignee:
No assignee Edit question
Solved by:
Nick Papior
Solved:
2019-12-04
Last query:
2019-12-04
2019-12-04
 Nick Papior (nickpapior) said on 2019-12-03: #1

1) Yes, this is so. Essentially you could also converge k-points along semi-infinite direction. However it is much simpler to just use a very high number (like 100)

2) Yes, they should be the same.
Sometimes it can work better to have k-points along the semi-infinite direction for the device. Other times not so much. In any case 1 along the semi-infinite direction in the device is fine.

3) This is done by analysing the k-averaged transmission for various k-samples. If this was what you meant, then yes.

Your final question is not specific to transiesta. This relates equally well to regular siesta calculations. So your choice.

 Tristan Zaborniak (tristanz) said on 2019-12-03: #2

Hi Nick. Thank-you for your response.

I have one further question: because TBTrans uses the output of a Transiesta calculation run at a particular bias, must we converge k-points in TBTrans for each bias we run a structure under?

 Nick Papior (nickpapior) said on 2019-12-04: #3

For consistency you should use the same TBtrans k-grid for all TBtrans calculations (otherwise comparing may be difficult).

 Tristan Zaborniak (tristanz) said on 2019-12-04: #4

Thanks Nick Papior, that solved my question.