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

[Lorenzo Paulatto]

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