[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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