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# Pulay mixing inversion failed. Using only last step.

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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.

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