# Pulay mixing inversion failed. Using only last step.

[Hasan Sahin]

After Electrodes Calculation and Equivalent Bulk Calculation (Initial Density for TwoProbe), during the TwoProbe Calculation

ATK sometimes gives the error

# Pulay mixing inversion failed. Using only last step.


and can not converge. How can I solve this ?

[Nordland]

This error can happen in two cases:

1) Either the results is soo well-converged - like 1e-16 - that the linear algebra behind the pulay mixing becomes numerical unstable
2) Or you results are soo messed up, that everything is lost

Since you are saying that, it is doing the TwoProbe SCF I would assume, and not during the equivalent bulk part, that
it is due to issues in the interface between the electrode and the scattering region. But of course I can't be sure, and
it is only my guess.

However when this error comes and you are not the the first guess, I would suggest looking at geometry and everything is correct,
the I would increase the accuracy of the energy contour integral parameters f.x by lowering the lower bound and the number of circle points.

[Hasan Sahin]

I increased history steps to 12, lowerbound to 4 and circle points to 50. ATK does not give the error but convergence still takes too long (and it may give error). I attached the geometry of the system. Does geometry script have mistake ?

[Nordland]

The geometry looks cool, and there is no critical errors at all.

I have tried to start a sample calculation on it on my laptop - trying out some parameters to see if I get the same problem as you.

[Hasan Sahin]

Now I am thinking about  electrode-device atom matching criterion... I hope one of us manage to calculate an I-V...Thank you for time and effort.

[Nordland]

I have now converged the system with Single Zeta and SingleZetaPolarized. I had some trouble using DZP, but I will try again now,
once this calculation is done, then I will post my parameters

[Hasan Sahin]

At the same time I did it.  It is impossible to converge it with DZ basis. It seems that voltage steps between two scf calculation must be smaller than 0.01 volt. I tried 0.02 volt but it can not find the initial density of next step by using scf of 0.00 volt.

This is a trial calculation with a very simple system, it seems that it is impossible to get I-V characteristics of larger systems. I am discouraged.

[Nordland]

I know the feeling! I have learn the hard way how much tougher it is to get NEGF to converge compared to usual DFT :|

However there is hope. I have done IV calculations on huge graphene sheets with 6000 atoms - so it is possible
And it was even with a gate voltage and I tried to model that these graphene sheets could be used a field-effect transistor,
and with good results.

The best way to get started on cracking these problems is for me, that starting out by a trial system, that involves the same chemical elements,
and geometries types as the target system. Then it is fast to try a few things and learn what makes this calculation converge fast and rock-solid.
Almost always can this insight be transfered to the target system right away.

I know there exist a little advanced tutorial on getting twoprobes to converge better and safer, but I dont have it.
Perhaps you can get it from QuantumWise if you write to them.....

[Nordland]

I got it working for DZP as well

[Hasan Sahin]

My calculations with SZ and SZP are ok.

1-)   I am curious about your DZP calculation (I couldnt get any I-V value with DZ and DZP).
1a-) How many different I-V values did you calculate?
1b-) Which parameters did you changed in your DZP calculation script?
 
2-) Do you have a published paper or a link about your graphene FET ?

[carbn9]

Hello Hassan,
the geometry you posted attracted me also since i'm also doing some calculations with carbon structures. i have some graphene geometries also on my web page: geocities.com/carbon89 if you r intereseted. but like u I did not achieve a I-V characteristics with polarized basis sets before. by the way, on which structures/subject are you working on??

[Nordland]

1-)   I am curious about your DZP calculation (I couldnt get any I-V value with DZ and DZP).
1a-) How many different I-V values did you calculate?
1b-) Which parameters did you changed in your DZP calculation script?

I tried a couple of things to be sure it converged, it has run smoothly, but I killed the calculation this morning when it had
done up to 0.45 volt in steps of 0.05 volt (10 seperate calculations), since I only have my laptop for doing these calculations

• 25 history steps with 0.05 diagonal mixing parameter
• 200 Ry mesh-cutoff ( most likely not needed )
• 1000 max iterations ( not needed since all voltages converged in 20-40 steps )
• (1,1,100) kpoints
• 600 Kelvin electron temperature(Improves alot, but does not do it alone)
• 0.05 eV real axis infitesimal(Improves alot, but does not do it alone)
• DensityMatrix electrode constraint(Best thing in ATK 2008.10 - apart from the speed-ups)
• 0.05 Volt steps

With these parameters It runs smoothly for DZP,  and I will post the IV-curve once I get the data processed.

2-) Do you have a published paper or a link about your graphene FET ?

I have not published the results :| - but let me see if I can find the scripts again, and then I can share them with you.

[Hasan Sahin]

Hello Hassan,
the geometry you posted attracted me also since i'm also doing some calculations with carbon structures. i have some graphene geometries also on my web page: geocities.com/carbon89 if you r intereseted. but like u I did not achieve a I-V characteristics with polarized basis sets before. by the way, on which structures/subject are you working on??

Hello carbn9,
Especially I am working on doped graphene flakes and graphene ribbons.

[Hasan Sahin]

1-)   I am curious about your DZP calculation (I couldnt get any I-V value with DZ and DZP).
1a-) How many different I-V values did you calculate?
1b-) Which parameters did you changed in your DZP calculation script?

I tried a couple of things to be sure it converged, it has run smoothly, but I killed the calculation this morning when it had
done up to 0.45 volt in steps of 0.05 volt (10 seperate calculations), since I only have my laptop for doing these calculations

• 25 history steps with 0.05 diagonal mixing parameter
• 200 Ry mesh-cutoff ( most likely not needed )
• 1000 max iterations ( not needed since all voltages converged in 20-40 steps )
• (1,1,100) kpoints
• 600 Kelvin electron temperature(Improves alot, but does not do it alone)
• 0.05 eV real axis infitesimal(Improves alot, but does not do it alone)
• DensityMatrix electrode constraint(Best thing in ATK 2008.10 - apart from the speed-ups)
• 0.05 Volt steps

With these parameters It runs smoothly for DZP,  and I will post the IV-curve once I get the data processed.

2-) Do you have a published paper or a link about your graphene FET ?

I have not published the results :| - but let me see if I can find the scripts again, and then I can share them with you.

Ok I will try with these parameters and upload the IV curve here . A calculation including 6000 atoms sounds good )).  Thank you for your time and fast informations.

[Nordland]

Finally I got around to getting some computer power to do the current calculation, as I had only did the SCF part first

For comparison I also calculated the linear response current to see the effect of the self-consistent response.
I have attached both the script and example of the IV curve for this system.

Cheers!