[CP2K-user] [CP2K:21964] Re: Reproducing NEGF results

[Michael LaCount]

Hello Dmitry,

My post was when I was initially learning the NEGF calculations. I have 
since solved the problems I was having, and reproduce the transmission 
function from the technical report. As well as producing what appear to be 
reasonable I(V) curves.

The I(V) curves I made are based on Eq 21 from the technical report using 
the zero-bias (ELECTRIC_POTENTIAL [eV] 0). Your comment "If the energy 
levels of the central system are not changed at finite voltage" leaves me a 
bit worried that I was not careful enough. Is there a simple way to test 
whether or not the assumption is valid? My guess would be something like 
apply a bias voltage up to the max of the I(V) curve and see if the 
transmission function changes significantly. What I am trying to simulate 
is a small semiconductor device. For simplicity sake lets say it has a band 
gap of 1 eV, and make I(V) curves in the range of +/- 3V. If the assumption 
brakes down how would I go about finding the I(V) curve, I could imagine 
finding the transmission function at various applied biases, but that seems 
like a computationally expensive approach.

I have been using a recent but not newest version of CP2K 2024.2. I am 
curious if the changes to the NEGF code are documented anywhere, and what 
impact they would have either in terms of performance or results.

Best,
Michael LaCount

On Friday, November 14, 2025 at 12:41:23 AM UTC-8 Dmitry Ryndyk wrote:

> Dear Michael, 
>
> to see the same result, as in the report, you should take zero voltage and 
> other energy limits:
>
> &PRINT
>      &DOS
>        FILENAME device
>        FROM_ENERGY 0.272240982
>        N_GRIDPOINTS 401
>        TILL_ENERGY 0.492829218
>
>      &END DOS
>      &TRANSMISSION
>        FILENAME transm
>        FROM_ENERGY 0.272240982
>        N_GRIDPOINTS 401
>        TILL_ENERGY 0.492829218
>      &END TRANSMISSION
>    &END PRINT
>
> The Fermi level is 0.38253510. The result is in attachment. It is not 
> exactly the same, but it can be for many reasons.  
>
> I am currently revising the NEGF code, and may change the energy levels to 
> be around the Fermi level. If you want to use NEGF, I recommend the latest 
> development version of CP2K. And I will try to answer further questions.  
>
> Concerning your second question. It is the advantage of the NEGF approach 
> that T(E) is changed with bias voltage. But as a simplest way to get I(V), 
> the zero-voltage transmission can sometimes be used. Very carefully to make 
> sense. It depends on the problem, of course. If the energy levels of the 
> central system are not changed at finite voltage, one can omit 
> self-consistency at finite voltage and use T_0(E).
>
> Best wishes,
> Dmitry Ryndyk
>
> Michael LaCount schrieb am Donnerstag, 10. April 2025 um 05:20:41 UTC+2:
>
>> I am trying to reproduce the transmission coefficient plot found in 
>> Figure 4 of the CP2K Electron Transport based on 
>> Non-Equilibrium-Greens-Functions Method: eCSE 08-09 Technical Report (see 
>> attached). From what I can tell the system is the same as the one found 
>> in 'QS/regtest-negf-fft/au111_c6h4s2_gamma_0.50V.inp'. However when I run 
>> that job unmodified except to add print commands in the NEGF section (see 
>> below) I don't get anything close to what was found in the Technical 
>> Report. The job input and output files are attached.
>>
>> I also have a further question regarding this test. I want to create I-V 
>> curves, from what I understand I should therefore use the zero bias 
>> transmission coefficients, and set NEGF/CONTACT/ELECTRIC_POTENTIAL to 0 
>> correct?
>>
>> Any clarity on where I am going wrong would be appreciated. 
>>
>>    &PRINT
>>      &DOS
>>        FILENAME device
>>        FROM_ENERGY -0.2
>>        N_GRIDPOINTS 401
>>        TILL_ENERGY 0.2
>>      &END DOS
>>      &TRANSMISSION
>>        FILENAME transm
>>        FROM_ENERGY -0.2
>>        N_GRIDPOINTS 401
>>        TILL_ENERGY 0.2
>>      &END TRANSMISSION
>>    &END PRINT
>>
>

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