Dear Guido and Giuseppe, Thank you both for your replies.
The structure should be ok, it represents an organic molecule between 2 gold electrodes. You should get something like the image I attach (do you?). It is a rather standard scheme, as you may also see in: http://pubs.acs.org/doi/abs/10.1021/ja2090096 although my structure is simplified for testing. I don't know whether ibrav=0 is strictly necessary but, anyway, it should not affect the outcome of the calculation, right? I have tested other k-point meshes without improvement. Anyway, I'll try denser meshes together with Giuseppe's suggestions and let you know the outcome. Cheers, Sergi 2015-02-03 13:56 GMT+01:00 Giuseppe Mattioli <[email protected]>: > > Dear Sergi > It seems that you bounce between more than one electronic configuration at > every scf step. This is far from uncommon in the case of molecular open > shell systems. :-( > I can add one or two things to Guido's good advice. > > > I've tried several tricks to try to improve electronic structure's > > convergence. Changing the 'mixing_beta', the 'type of mixing', removing > the > > smearing, > > Use always the more robust gaussian smearing (at least 0.01 Ry, but also > more if you struggle for convergence) for geometry optimizations. If it does > not converge, then m-p and m-v will also fail 99.9% . You can use other > schemes after you tamed the system. Reduce also the mixing_beta down to 0.05 > or 0.02 (with davidson). > > Maybe your starting configuration is horribly unlucky. Try (carefully) to > use something like > > conv_thr=1.0d-5~1.0d-6 > electron_maxstep=50~100 > scf_must_converge=.false. > > and see if your convergence problem improves after few bfgs steps without > showing unphysical behavior. > > If your SOMO is higher than the Ag Fermi energy, then there is probably no > way to have an integer value for the magnetization of the system because of > the molecule-to-surface charge transfer. You may try not to bias the > tot_magnetization variable. > > Finally, ecutwfc=25.0 seems a bit too low. It could be a source of > instability. I would use something around 35/280 Ry for ecutwfc and ecutrho. > > HTH > Giuseppe > > > On Tuesday, February 03, 2015 10:57:23 AM Sergi Vela wrote: > > Hi all, > > > > I'm having a lot of troubles in modeling the adsorption of molecules on > > surfaces. I've quite a lot of experience with QE when working with > crystals > > made of open-shell molecules but I'm not familiar with the description of > > metal surfaces. > > > > First of all, I'm using Qespresso v.5.1.1 > > > > I succeed in simulating 4 layers of a 111 Gold surface with Quantum > > Espresso (QE). However, using the same surface and unit cell, when I > > include a magnetic (open-shell) molecule on top of that surface, even if > > its far away from it, I cannot reach convergence. I'm 99% sure that the > > input is fine in terms of the unit cell definition (see input file > > attached) and the problem is just the convergence of the electronic > > structure. > > > > I believe the problem is the fact that the system is highly inhomogeneous > > as it has a metal surface and an open-shell molecule. I take as an > example > > the smearing, since, although it is mandatory when modeling the metal > > alone, it seems to hinder the description of an open-shell molecule. > > Originally, I used methfessel-paxton smearing with a 'degauss' of 0.2 eV. > > The convergence seems to be even worse as the value is increased, and > > changing the type of smearing doesn't help neither. > > > > I've tried several tricks to try to improve electronic structure's > > convergence. Changing the 'mixing_beta', the 'type of mixing', removing > the > > smearing, switching to 'cg' algorith, increasing number K points ... all > of > > them with identical output (take Summary.out as an example of the typical > > evolution of the SCF energy, you'll see that is awful). > > > > Anyone has experience on modeling such king of systems? > > > > Any help is appreciated, thanks in advance > > > > Dr. Sergi Vela, > > University of Strasbourg, France. > > ******************************************************** > - Article premier - Les hommes naissent et demeurent > libres et ègaux en droits. Les distinctions sociales > ne peuvent être fondèes que sur l'utilitè commune > - Article 2 - Le but de toute association politique > est la conservation des droits naturels et > imprescriptibles de l'homme. Ces droits sont la libertè, > la propriètè, la sùretè et la rèsistance à l'oppression. > ******************************************************** > > Giuseppe Mattioli > CNR - ISTITUTO DI STRUTTURA DELLA MATERIA > v. Salaria Km 29,300 - C.P. 10 > I 00015 - Monterotondo Stazione (RM) > Tel + 39 06 90672836 - Fax +39 06 90672316 > E-mail: <[email protected]> > http://www.ism.cnr.it/english/staff/mattiolig > ResearcherID: F-6308-2012 > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum >
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