[Pw_forum] bfgs optimization not going anywhere

Ari P Seitsonen Ari.P.Seitsonen at iki.fi
Tue Nov 24 11:21:50 CET 2015

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,


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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