[QE-users] DFT+U and surfaces -- convergence

[Cococcioni Matteo]

Dear Roberto,

in addition to what Lorenzo has already suggested, one more thing to try is to switch on the value of U in small steps. you start from a small value, e.g. 0.01 eV which should go to convergence as smoothly as a plain DFT run does, then you read potential and wavefunctions and start another run at a bit larger value of U, converge it, restart at a larger value of U and repeat until you reach the value you want to use.
In many case where convergence is difficult this helps (even though it may take some steps).

Hth.

Best,

Matteo




On Jul 1, 2018, at 1:45 PM, Lorenzo Paulatto <paulatz at gmail.com<mailto:paulatz at gmail.com>> wrote:

On 06/30/2018 04:28 PM, Roberto Gomes de Aguiar Veiga wrote:
However, when I try to perform calculations for the corresponding surfaces, they do not converge. I've been using mixing_mode='local-TF' and descreasing mixing_beta to values as low as 0.05 for the surface calculations, with no success. Can any of you with experience in this kind of simulation (DFT+U and surfaces) share what you usually do to have converged calculations?


Hello Roberto,
DFT+U and surface is a bit tricky, because in principle there is not guarantee that the value of U used for the bulk is transferable to the surface atoms. We all do it, but it is not justified.

I also find it very difficult to converge DFT+U for surfaces. One approach I have used is to defined different species for atoms of the uppermost layers and for deeper atoms, to be able to define different starting magnetization. Also, I do not refrain to increase electrons max steps to 1000 or more, sometimes it just take that long to converge!

Finally, a trick which I have used is to use starting_ns_eigenvalue and mixing_fixed_ns to some large value (e.g. 1000) to get a first converged charge density with ns eigenvalues kept fixed, that do a restart="from_scratch" with startingpot="file" (this time without mixing_fixed_ns, or setting it to a small value).

If you notice that the ns eigenvalues printed in output are less symmetric than the initial magnetization, it may be a good idea to split the definition of the +U species until they do, this way you can define different initial ns for them.

hth, it is a lot of cooking



--
Lorenzo Paulatto - Paris
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************************************************
Matteo Cococcioni
Theory and Simulation of Materials
École Polytechnique Fédérale de Lausanne






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