Author Topic: # Pulay mixing inversion failed. Using only last step.  (Read 15137 times)

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Offline Hasan Sahin

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# Pulay mixing inversion failed. Using only last step.
« on: December 28, 2008, 12:07 »
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 ?

Offline Nordland

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #1 on: December 28, 2008, 12:54 »
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.

Offline Hasan Sahin

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #2 on: December 28, 2008, 15:42 »
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 ?

Offline Nordland

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #3 on: December 28, 2008, 16:23 »
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.

Offline Hasan Sahin

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #4 on: December 28, 2008, 16:47 »
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.

Offline Nordland

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #5 on: December 28, 2008, 20:15 »
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 :)


Offline Hasan Sahin

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #6 on: December 28, 2008, 20:33 »
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.

Offline Nordland

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #7 on: December 28, 2008, 20:50 »
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.....


Offline Nordland

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #8 on: December 28, 2008, 22:13 »
I got it working for DZP as well :)

Offline Hasan Sahin

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #9 on: December 29, 2008, 11:39 »
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 ?

Offline carbn9

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #10 on: December 29, 2008, 11:58 »
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??

Offline Nordland

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #11 on: December 29, 2008, 12:57 »
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.

Offline Hasan Sahin

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #12 on: December 29, 2008, 13:02 »
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.

Offline Hasan Sahin

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #13 on: December 29, 2008, 13:08 »
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.
« Last Edit: December 29, 2008, 13:16 by Hasan Sahin »

Offline Nordland

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Re: # Pulay mixing inversion failed. Using only last step.
« Reply #14 on: December 29, 2008, 17:24 »
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!