[Pw_forum] Units of Charge Density - Again

[Tone Kokalj]

On Thu, 2008-09-18 at 09:34 +0200, Lorenzo Paulatto wrote:
> > Finally, I've been unable (by visual inspection) to determine from the
> > documentation for XcrySDen (a marvelous tool...marvelous), exactly how
> > the "rho grid input" is scaled, especially regarding the computation
> > of the 3D min-max grid values, the "isovalue"...and...particularly the
> > mysterious "delta n(r)" displayed in the temperature inset box.

This \Delta n(r) label is mysterious indeed, and is there solely for
convenience: if one plots the charge density difference, then one would
like to have "\Delta" sign, but it is not possible to input it with
keyboard (unless one knows its ascii code), so I put it there. If one
plots total charge density or some other property like potential, then
this label can be simply deleted and replaced  by the corresponding one.

> >
> I don't know if it's scaled at all, I think it is at most a side effect
> of interpolation. If you use the minimum level of interpolation do you
> get the extremes you expected? Anyway, I think it is better to wait for
> Tone to reply on this topic

Here I am (I was off-line previous week). 

Xcrysden does not assume any unit of charge, and simply leaves the values 
as stated in the file. In PW the unit is e/bohr^3, so this unit remains intact 
in xcrysden (this means that in this case the unit is e/bohr^3, e being the electron charge).

The fact that the spanning vectors + origin for the grid 
are specified in Ansgtroms does not affect the unit of charge at all. 
This data is merely used to locate the grid points
in the space. They could be equally well represented in meter or bohr units:
the position of points would still be the same, and at each point the value of charge as 
supplied by the file is used.

Regards, Tone
-- 
Tone Kokalj <tone.kokalj at ijs.si>
J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia (tel:
+386-1-477-3523 // fax:+386-1-477-3822)