TranSIESTA: Calculating I-V curve using 4.1-b3

Asked by Tristan Zaborniak on 2019-10-07

Having been a previous user of the 4.0 series of Siesta/Transiesta, I've been transitioning into using the 4.1 series, but am finding it difficult to understand some things.

In the 4.1-b3 user manual, it is written that 'it is important to realize that the parameterization of the voltage into the chemical potentials enables one to have a single input file which is never required to be changed, even when changing the applied bias'.

My first question to this is: how? Can we now run Transiesta something like: Transiesta -v 1.0 input.fdf < output.fdf if we wanted to apply a 1.0 eV bias to our scattering region? What of the TS.Voltage flag? Doesn't this determine the bias of the system, and, if it does, doesn't that mean that one must change this parameter *in* the fdf in wanting to change the applied bias of the structure?

This is confusing, as within the chemical potential blocks for a two-electrode system, mu is set in one to -V/2 and in the other to V/2 by ts2ts, but nowhere is V defined explicitly! Previously, in the 4.0 series, one had to change the TS.Voltage flag in the input file between runs at different biases.

Also, in running the 4.1-b3 version of TBTrans, to get the current through the (two-terminal) system as a function of voltage, is it necessary to run TBTrans once for each test bias? The TBT.HS.Files block makes it seem as if this may be unnecessary, and that one can calculate an I-V curve in running TBTrans once. And, where is the current output? In the 4.0 series, this was in the TBTrans .out file, but this doesn't seem to be the case with the 4.1 series...

Thank-you for your help.

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Nick Papior
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Best Nick Papior (nickpapior) said : #1

1) Don't use 4.1-b3 (it is buggy), 4.1-b4 is out!

2) Your note about what is written in the manual. This is because in 4.0 one had to change the TS.biasContour.NumPoints for each bias to get a consistent calculation. In 4.1 the bias contour is parameterized using an energy spacing and thus one can use the same input for all applied bias'. You can either do:

siesta -V 0.1:eV RUN.fdf

or manually change TS.Voltage in the fdf file. Both are equivalent (the command line however has precedence!).
They are equivalent methods of doing the same thing. (I'll update the manual regarding this).

3) Yes, you need to run tbtrans for each individual bias. The point about the TBT.HS.Files block is that you can interpolate the Hamiltonians for different applied bias' and thus *estimate* the true I-V curve without having to do transiesta calculations for all intermediate bias'.
No where in the manual does it state you get an I-V curve from a single tbtrans calculation. You simply can't.

4) As for why the current is not output in the 4.1 tbtrans version, please see:

Tristan Zaborniak (tristanz) said : #2

Thanks Nick Papior, that solved my question.