[Pw_forum] bfgs optimization not going anywhere

[Paolo Giannozzi]

Just an observation: there are constrained atoms. I am not sure how well
current convergence criteria for BFGS deal with this case.

Paolo

On Tue, Nov 24, 2015 at 11:37 AM, Matej Huš <matej.hus at gmail.com> wrote:

> Thanks for very useful comments.
>
> Yes, vacuum region is indeed too low in this example (whoops, seems like
> I've attached an old input file). I also ran a calculation with 10 A of
> vacuum without any improvement. Open shell is unnecessary as it quickly
> converges to a closed shell solution. I've just wasted some CPU time with
> that.
>
> Ectuwfc, ecutrhc are large enough (converged). I believe K points should
> also not be the source of a problem.
>
> As for DFT-D2, I admit I am a bit in the dark here. How 'wrong' is it? Or
> to
> put differently, is it more wrong that not using dispersion correction at
> all? How about vdw-DF, how do I turn it on and does it perform better?
> Where
> can I find more important about that?
>
> Also, if I change/switch off the dispersion correction, then I will have to
> re-optimize all already converged structures (metal, isolated molecules,
> adsorbed intermediates), right? Looks rather tedious
>
>
> Matej
>
>
>
> -----Original Message-----
> From: pw_forum-bounces at pwscf.org [mailto:pw_forum-bounces at pwscf.org] On
> Behalf Of Ari P Seitsonen
> Sent: Tuesday, November 24, 2015 11:22 AM
> To: PWSCF Forum <pw_forum at pwscf.org>
> Subject: Re: [Pw_forum] bfgs optimization not going anywhere
>
>
> Dear Matej Hus,
>
>    Adding to the previous comments (I think that it is ok to use the k
> points, even though you might gain some CPU time by first indeed using
> Gamma-only and then increasing, as your cell is quite large), have you
> checked the length of your vacuum/lattice vector along the surface normal?
> If I see correctly, you have only about six Ångströms, which is already
> very
> (too) little, plus the DFT-D2 (Grimme-D2) adds quite strong an interaction
> through the vacuum at such distances I believe, plus the dipole
> correction...
>
>      Greetings from Peaceful Montrouge/France,
>
>         apsi
>
> PS My personal opinion of the vdW-DF* is, yes, they might be better, but
> first check the literature about a "suitable empirical choice of the
> particular mixture" of the functional, as some of the choices give quite
> large lattice constants already (4-5 % larger than experimental, which, of
> course, is not necessarily the target, rather a good adsorption geometry
> and
> energy). And often the best agreement is found by tuning the exchange
> functional, which naturally should have not nothing to do with the
> vdW/London dispersion, which is pure correlation... So much about "ab
> initio". ;)  The good news of the day is that it seems that seldomly things
> seem to go completely wrong, no matter which choice of the vdW-DF* you
> take,
> but still, I would study a bit the literature first. If any one has a
> reference for a good review on the vdW-DF* et co applied to surfaces and
> adsorption, I would also be very thankful! :)
>
>
> -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=
> -
>    Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
>    Ecole Normale Supérieure (ENS), Département de Chimie, Paris
>    Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935
>
>
> On Tue, 24 Nov 2015, Matej Huš wrote:
>
> >
> > Dear PW community
> >
> > I have a problem when trying to find optimized geometry of an adsorbed
> > intermediate in "benzene" hydrogenation on Ru surface (see input below).
> >
> > I believe I am fairly close to optimised structure since I'm only
> > adding one hydrogen  atom to an already optimised reactant and only
> > slight displacing the adjacent atoms.
> >
> >
> >
> > The problem is that BFGS doesn't seem to go anywhere. Energy only
> > fluctuates, which to my understanding is a sign of being close to a
> minimum.
> > However, forces remain large.
> >
> >
> >
> > I've switched to the damp algorithm and the problem persists. I've
> > searched through the forum and the usual approaches of tackling this
> > issue do not seem to work.
> >
> >
> >
> > Any ideas? Thank you!
> >
> > Matej Hus
> >
> > Laboratory of Catalysis and Chemcial Reaction Engineering
> >
> > National Institute of Chemistry
> >
> > Ljubljana, Slovenia
> >
>
>
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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