[Pw_forum] bfgs optimization not going anywhere

Tue Nov 24 14:30:46 CET 2015

Dear Matej (and all)

> 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?

Regarding the dispersion correction(s) this is the Stephan Grimme's paper which indicates that DFT-D2 doubles the DFT-D3 interaction energy between 
molecules and metal surfaces:

J. Chem. Phys. (2010) 132, 154104

In my experience of phthalocyanine/metal interactions (but if I remember well Ari has also performed calculations on porphyrin/phthalocyanine 
molecules adsorbed on metal surfaces and he can add his own knowledge), adsorption geometries and energies are very different, so that the results are 
also quite different. See for example this paper (Cu phthalocyanine interacting with Ag (100) with DFT-D2)

PRL 105, 115702 (2010)

And my paper (TiO phthalocyanine interacting with Ag (100) with vdW-DF)

J. Phys. Chem. C 2014, 118, 5255

And, yes, of course: if you use no dispersion correction you badly underestimate the interaction between the molecule and the surface...

2 more hints:

1) In the case of molecule/metal systems, ab initio functionals modeling long distance correlation between electrons should be (in my opinion) 
preferred to semiempirical functional with add pairwise forces based on tabulations (or partial calculations) of C6 coefficients. There is a nice 
perspective written by J. L. Bredas which addresses the issue

    Nature Nanotechnology
    8,
    230–231
    (2013)

2) Regarding lattice parameters, Ari is right in the case of vdwdf. But I've tried to simulate gold (bulk and) surfaces in contact with molecules by 
using the reparametrized vdwdf-c09 functional: lattice parameter of gold is ok, and also molecule-surface interactions are expected to be improved 
with respect to vdwdf!

> 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

Yes, you should recalculate everything...:-(

HTH
Giuseppe

On Tuesday, November 24, 2015 11:37:45 AM Matej Huš 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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