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.seitso...@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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