CHGCAR & Guidebook: what is Vcell?

[lahaye]

Hello,

The guidebook says on CHGCAR:

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...the total charge density [is] multiplied by the volume rho(r) * Vcell
on the fine FFT-grid (NG(X,Y,Z)F). 

and

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 Please do not forget to divide by the volume before visualizing the
file. 

From this I suppose that what is meant by Vcell here is the volume
spanned by the three lattice vectors from POSCAR, devided by
NGXF*NGYF*NGZF.

For example:

POSCAR has:

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 0.5
  24.0 0.0  0.0
  0.0 24.0  0.0
  0.0 0.0  2.0 

and NG(X,Y,Z)F = 200, 200, 20
then

Vcell = (0.5)^3 * 24*24*2 / (200 * 200 *20) = 0.00018 A^3.

and for visualization of the charge density, I must devide each density
value in CHGCAR by 0.00018 A^3.

Is that right?

Thanks,
Rob.

[bandy]

I think vol of the cell in your case is 144 Angstrom^3. The charge density rho(NGX,NGY,NGZ) should be read from CHGCAR and then multiplied by Vcell to display charge. I am interested to know what others say.

<span class='smallblacktext'>[ Edited Tue May 16 2006, 12:16AM ]</span>

[lahaye]

Thanks bandy for your reply to my question.

bandy wrote:

I think vol of the cell in your case is 144 Angstrom^3.

OK, so "cell" means the whole box as specified in POSCAR.
I thought (wrongly) that "cell volume" here meant the tiny cell
of the FFT-grid.
Anyway, if you're right on this point, then that clarifies already
this bit for me.

bandy wrote:

The charge density rho(NGX,NGY,NGZ) should be read from CHGCAR and then multiplied by Vcell to display charge.

I'm almost sure that you're wrong here.
The Guidebook says explicitly, that the values in the CHGCAR file are
the charge density mulitplied by the volume cell Vcell; and that
you must divide by this volume to obtain the charge density for
visualization.

By the way, the charge density rho(r) is not on the (NGX,NGY,NGZ) grid,
but on the much finer (NGXF,NGYF,NGZF) grid, as far as I understand.

I hope someone else can clarify a bit more on this matter.

Rob.

[bandy]

So far I know NGXF.. are for augmentation charge density, maximum it could be 30% more depending upon ENCUT and PREC setting.
<span class='smallblacktext'>[ Edited Wed May 17 2006, 01:00AM ]</span>

[admin]

1) yes, the CHGCAR file contains charges (rho * V), not charge densities.
2) yes, the grid density on which the charges are written is the NGXF,... one, (compare line 13 of CHGCAR with the FFT-grid data written in OUTCAR to verify that).
you can find a nice table showing the relation between the different grids (depending on PREC and ENCUT) on p25 of the accuracy.pdf tutorial file of the vasp-workshop
http://cms.mpi.univie.ac.at/vasp-workshop

[caster29]

Dear Users,

After these posts, I still don't understand something!

What is the meaning of V (or Vcell)?

Is it the total volume of the cell in real or in reciprocal space??????

I have another question concerning the units in this file.

Is rho given in Coulomb.m-3 and consequently "Rho.V" is in Coulomb?

Thanks in advance,

Damien

[admin]

Vcell is the volume of the cell in real space (as defined by the Bravais matrix in POSCAR). rho is given in numbers of electrons (the unit of charge if you work in atomic units), charge densities are given in electrons per Ã…3