[CP2K-user] [CP2K:12058] Re: Cell Optimization does not work in QM

[Pierre Cazade]

Hi Matt,

Thanks for the tip about the cell_ref, I'll give it a try. Otherwise, I 
define the symmetry and I keep it fixed. I also keep the angles fixed 
for the first optimization.

A new calculation I've just run with the DFT parameters from Dirk's 
input file yields similar results as I obtained before:

  CELL| Vector a [angstrom]:       4.251     0.000     0.000    |a| 
=       4.251
  CELL| Vector b [angstrom]:       0.000     7.405     0.000    |b| 
=       7.405
  CELL| Vector c [angstrom]:      -2.287     0.000     5.298    |c| 
=       5.771

I started with this however:

&CELL
       ABC [angstrom]   5.078       6.192     5.387
       ALPHA_BETA_GAMMA [deg] 90.00 113.35  90.00
       SYMMETRY MONOCLINIC
       PERIODIC XYZ
&END CELL

I know from VASP the optimized unit cell is:

    a                  b             c         alpha beta          gamma
  5.12269  6.41270  5.41981   90.0000    112.7275     90.0000

Of course, I do not expect two different software using different 
methods and different pseudopotentials to give the exact same result but 
they should be close enough. The contraction of the A vector and the 
expansion of the B vector are absurd. I will try the ref_cell, it might 
help.

Regards,
Pierre


On 08/08/2019 11:38, Matt W wrote:
> Standard inputs are in Angstroms for a coord section.
>
> The most common problem is that the planewave basis changes as the 
> cell optimizes. You can use a constant size of plane wave basis by 
> specifying a reference cell, something like this. Where the CELL_REF 
> contains a reference cell that should be a bit bigger that the 
> expected largest cell after optimization.
>
>      &CELL
>        ABC 5.620 5.620 5.620
>        &CELL_REF
>          ABC 5.620*1.1 5.620*1.1 5.620*1.1
>        &END
>      &END CELL
> If your cell has symmetry (orthorhombic, hexagonal etc) you can 
> specify this to be kept. Similarly first optimizing just the volume 
> and keeping the angles constant, then relaxing everything can be more 
> efficient.
>
> Also I'd stick with BFGS rather than LBFGS unless you system is huge 
> (> 2000 atoms)
>
> Maybe this helps a little,
>
> Matt
>
>
> On Thursday, August 8, 2019 at 10:46:18 AM UTC+1, Dr. Dirk Buddensiek 
> wrote:
>
>     Hi Pierre,
>
>     good luck. Concerning distance units: It might be that there are
>     in Bohr for MP2. But you can set the units explicitly in the input
>     deck.
>
>     Dirk
>
>     Am Mittwoch, 7. August 2019 17:05:56 UTC+2 schrieb Pierre-André
>     Cazade:
>
>         Hello,
>
>         I am rather new to CP2K though I am experienced with molecular
>         modelling both quantum and classical, and with a wide range of
>         software. It is quite frustrating that it seems impossible to
>         get CP2K to perform a simple CELL_OPT of a beta glycine
>         crystal (20 atoms, monoclinic) with PBE. Such a calculation
>         works fine with VASP, so I am quite surprised the same
>         calculation goes astray with CP2K. Basically, the B unit cell
>         vector increases drastically up to absurd values. Needless to
>         say, this is a test system as I wish to use CP2K for its
>         linear scaling DFT to model protein crystals. I have tried to
>         play with CUTOFF and REL_CUTOFF in the MGRID section up to
>         1400 and 80 respectively. I also tried GPW and GAPW methods,
>         all electrons or GTH pseudo. Nothing works. Please, if anyone
>         could have a look at the attached input file "cell_opt_qm.inp"
>         and point what's wrong in it, I would be grateful.
>
>         Furthermore, I have tried LS approach with DFTB on a 5x5x5
>         crystal of beta glycine and do not have much more success.
>         First, the pressure in the crystal is insanely high and will
>         lead again to a 20 folds increase of the lattice vectors (with
>         CG). Then, with LBFGS method, the system stops after 3 to 4
>         cycles with a message telling LBFGS convergence criteria were
>         reached, whereas the system is clearly not. Again, I would be
>         grateful if anyone could tell me what's wrong with my system
>         ("cell_opt2_qm.inp" with "betacry.pdb" for the coordinates).
>         Ideally, my goal would be to be able to run large complex
>         systems with the LS approach and with any DFT/DFTB method.
>
>         Thank you in advance for your help.
>
>         Regards,
>         Pierre
>
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-- 
Dr Pierre Cazade, PhD
AD3-023, Bernal Institute,
University of Limerick,
Plassey Park Road,
Castletroy, co. Limerick,
Ireland

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