[Pw_forum] modeling open-shell molecules on metal surfaces

[Sergi Vela]

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 <giuseppe.mattioli at ism.cnr.it>:

>
> 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
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