# [CP2K-user] [CP2K:15340] Ar lattice constant calculation

Fabian Ducry fabia... at gmail.com
Wed May 12 08:26:56 UTC 2021

```Dear Sumit

A supercell of (2x2x2) has to contain 2*2*2=8 times the number of atoms
of the unit cell. So It can never have an odd number as in your case 63.
You should follow a tutorial on how to prepare such a structure, e.g.
with ASE. Here, for example, you find instruction on how to build a
crystal: http://exciting-code.org/lithium-atomic-simulation-environment.
Once you have the ASE Atoms object building the supercell is as easy as
atoms*(2,2,2). When you have a correct crystal, make sure that the &CELL
section in the cp2k input matches the lattice dimensions of the crystal.
You cannot just use an arbitrary number.

CELL_OPT will optimize both the atom positions as well as the lattice
dimensions.

Fabian

On 12.05.2021 06:19, sumit agrawal wrote:
>
> Dear Lucas Lodeiro and Fabian,
>
> Thanks for your nice suggestions. I am new to cp2k. I have just
> started learning this. I have some query regarding my procedure for
> preparation for crystal structure. I am following below procedure to
> prepare the crystal structure:
>
> First, I have prepared the crystal structure (2x2x2) with the lattice
> dimension a=5.2229 A, b=5.2229 A and c=5.2229 A. It contains 63 atoms.
> After that I removed some atoms from x, y and z directions so that it
> become periodic in x, y and z direction. After this, it contains 32
> atoms.
> Now I save this lattice in XYZ format. This will be my
> initial coordinate system. Am I correct?
>
> After this I took this coordinate file and ran for the cp2k calculation.
>
> Now in cp2k calculations for this lattice structure what will be my
> cube size? (Here you mention it will not be 20 A. )
>
> One more query, here I am starting with a lattice dimension with
> 5.2229 A . So after Cell_OPTIMIZATION, will the geometry of my crystal
> change or not?
>
> Yes, dispersion correction must be required for this type of
> calculation, I will do that.
>
> Here I am attaching my crystal structure images.
>
>
>
> Thanks
> Sumit
>
>
>
> On Tue, May 11, 2021 at 11:02 PM Lucas Lodeiro <eluni... at gmail.com
> <mailto:eluni... at gmail.com>> wrote:
>
>     Hi Sumit,
>
>     First of all, you cannot find the lattice constant with this type
>     of calculation. You ran an ENERGY calculation. If you want to do
>     it, you need to run a CELL_OPT calculation.
>     In your current calculation, you use a 20 A lattice constant cube.
>     Also, I think your initial cell dimension is not correct, since
>     the lattice constant of Argon is 5.26 A, then a 2x2x2 supercell is
>     not 20 A sized. Also, your system has 63 atoms... ¿?.
>
>     Other related topics are: a 2x2x2 supercell is not sufficient to
>     compute a periodical Argon system... You need a bigger supercell,
>     or use kpoints. Also, as this system just interacts by dispersion,
>     probably the PBE functional is not sufficient at all. You need to
>     account the dispersion terms by means of Grimes D3 or other.
>
>     Regards
>
>     El mar, 11 may 2021 a las 14:08, sumit agrawal
>     (<sumitag... at gmail.com <mailto:sumitag... at gmail.com>>)
>     escribió:
>
>         Hii everyone,
>
>         I want to calculate a lattice constant for the Ar crystal of
>         (2*2*2 unit cell). I am attaching my input and out files.
>         Where I will find the lattice constant ?
>
>
>
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