[CP2K:5050] Re: restarting from an already optimized structure

SRKC Sharma Yamijala sharma... at gmail.com
Sat Mar 8 09:39:06 UTC 2014

Dear Dr. Flo,

Thanks a lot for the reply. My main aim of this project is to study the
gold clusters on graphene quantum dots (GQDs) and compare the energies of
different conformers of gold clusters on these GQDs. I have varied the GQDs
from monolayer (290 atoms) to trilayer (830 atoms) in the studies. I have
attached the coordinates of GQD+Au cluster for the input which I have given
in the previous post (name of the file doesn't represent the original
system, though).

I took 38*38*38 box to avoid any interaction between the periodic images
and I also used Poisson solver to truncate the wavefunction at the edges
for the same reason.

*If it is useful:* My main aim is to check which gold conformer is more
stable on a GQD. When I checked the difference in the energies (between two
conformers) by varying the cutoff from 320 to 500 Ry it didn't change more
than 0.01 eV, and hence, I got satisfied with 320 Ry. But, I am also
suffering from faster convergence of the calculations. Indeed the
convergence issues are becoming more problematic when I am considering
bigger systems (i.e. bi-layer or tri-layer GQDs). But, I have also observed
that, with the same 320 Ry cutoff, the PBE calculations converged pretty

Thanking you for your valuable time,
Chaitanya Sharma.

*Chaitanya Sharma,*
*Prof. Bala and Prof. Pati'*s groups,
Chemistry and Physics Materials unit,
Lab:: (080-2208) 2581, 2809

On Sat, Mar 8, 2014 at 2:25 PM, Florian Schiffmann
<flosch... at gmail.com>wrote:

> I see the problem. You have a farily big cell (38x38x38) and going to high
> cutoffs is likely to cause a memory issue. The problem is, BLYP
> pseudopotetnials need very high cutoffs to be smooth with the FFT's. I
> don't know what your system looks like exactly but the only way out here as
> far as I can tell is to use a smaller cell if you can't afford the settings
> as they are. I don't know what the system looks like (dimensionwise) but it
> might be possible to use MT and 2D periodic boundaries with a smaller box
> permitting you to use higher cutoffs. Do you mind sending me a snapshot or
> position file?
> If you don't use the center coordinates keyword with the final geometry
> the positions should be fine (and optimal at least with the PW basis used
> but wrong compared to the basis set limit). Furthermore if you want to do
> energy comparisons (reactions or whatever) you definitely need a better
> convergence of your PW basis otherwise there will be a position depndent
> (relative to the gridpoints) error in all your energies.
> About the flucuations in the energy. BFGS is very sensitive to the ripples
> in the PES and only behaves well if they are down to a reasonable magnitude
> (see above more expensive but better convergence).
> Flo
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