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

[Giuseppe Mattioli]

And on top of all these valuable suggestions you may try the desperate  
(but often wise in my experience) trick:

     electron_maxstep=50~100
     scf_must_converge=.false.,

This is very useful when you want to optimize a structure and your  
atomic configuration is far from equilibrium (e.g. a surface starting  
from a cleaved bulk). But you must carefully check that your system  
does not go bananas...

HTH
Giuseppe

Quoting Cococcioni Matteo <matteo.cococcioni at epfl.ch>:

> 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



GIUSEPPE MATTIOLI
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