[Pw_forum] modeling open-shell molecules on metal surfaces

[Giuseppe Mattioli]

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.

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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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