# What is the transiesta+tbtrans procedure?

I have tried to run some calculations on transiesta/tbtrans with a non-zero voltage. I started with a zero voltage calculation and copied the .TSDE file to run a calculation with V=0.25. However, I still receive the error: You have to calculate the 0 V and re-use the TSDE from that calculation. So, should the calculations be executed like this:

1. electrode to produce the TSHS for the bulk electrode (siesta --electrode).
2. Use the TSHS electrode file to calculate the 0V TSDE+TSHS scattering region with transiesta (siesta executable).
3. Use the TSHS and TSDE scattering region files (and the electrode TSHS) to compute the 'final' properties at non-zero voltage (only tbtrans executable).

Lastly, even if the tbtrans calculation finishes without errors, no 0_Normal_exit file is produced?

Thanks!

• I changed the title, @Laura, please advice if you think the new title is not correct. Commented Nov 28, 2022 at 12:09

## 1 Answer

You got it a bit wrong.

## TranSiesta

This program (a sub-module in Siesta) self-consistently solves the NEGF equations to calculate the system at a finite bias. Upon self-consistency, the Siesta program will write out the Hamiltonian under the effect of the bias as given in the input file.
This effectively means that for each bias-point one needs to self-consistently solve the NEGF equations to obtain the true electronic structure for a given bias point.

Since the electrodes are in equilibrium (even under finite bias) one can use the same electrode Hamiltonian for all bias points, by definition!

## TBtrans

This program can calculate elastic transport by inputting the electronic structure of the electrodes (still under equilibrium) and the device region for a given bias point. If the electronic structure for the device region is solved for a given bias $$V$$, then TBtrans can only correctly calculate the transport for that same bias $$V$$, if you give TBtrans any $$V'\neq V$$ your calculation will be wrong.

What TBtrans can provide is a way to interpolate the electronic structure of several bias-points (several TSHS files calculated with TranSiesta for different bias points) and thus give an estimate of the true transport characteristics for a bias point where the electronic structure has not been explicitly calculated. This interpolation should of course be done with care and before-hand knowledge of the electronic structure should give you details about adequate interpolation ranges. Say for one system you can interpolate in a range of 0.5 V, whereas for another system, you can only interpolate in a range of 0.25 V.

TBtrans does not create the 0_NORMAL_EXIT file.

## The procedure

1. Calculate the electrode electronic structure
2. Calculate the 0 bias case using TranSiesta
3. Calculate any non-bias cases using TranSiesta and by using the closests bias point previously calculated using TranSiesta (copy that TSDE file). Be sure to set the flag: DM.UseSaveDM T in your input.
4. Finally, calculate the transmission for the bias points done in steps 2 and 3
• Dear Nick, thanks for the answer! I continued receiving the error even by following your step-by-step explanation. However, I noticed that my error was due to the fact that one should explicitly include the DM.UseSaveDM to true in the transiesta fdf file. Thanks again Commented Nov 29, 2022 at 8:29
• @Laura that is true. I have amended the answer with your comment. Commented Dec 7, 2022 at 12:11