# [Pw_forum] what's the unit of charge density

Tone Kokalj tone.kokalj at ijs.si
Tue Apr 14 12:15:11 CEST 2009

On Sat, 2009-04-11 at 15:15 +0800, vega lew wrote:
> Dear all
>
> I just ploted the 3D charge density using pp.x code.  And XCRYSDEN was
> used for displaying the 3D isosurface of charge density. When I set
> the isovalue for different number, such as 0.15 or less, I fould the
> overlap of electron cloud is quite meanful for different systems of
> distinct catalysis activities. I wonder what the unit of charge
> density?

xcrysden does not assume any units, and merely takes the values as
stated in the file. The unit is therefore up to the creator of the XSF
file.

If the XSF file was produced by pp.x, then the unit of charge is
electron/bohr^3.

> and whether the absolute value of it could be directly comparable for
> different systems ( same number of atoms, same element, only different
> in geometry of the model)?

Yes, because the unit is electron/bohr^3 (for pp.x).

> By the way, is there a tutorial for population analysis? I wonder
> where I could find the value of overap population for a pair of
> atoms.

The PWscf uses Lowdin population analysis that can be performed by
projwfc.x. Note however the ingredients are lowdin oribtals, which are
orthonormal, i.e.:

LowdinOrb_\mu = \sum_\nu S_{\mu,\nu}^{1/2} AO_\nu,

where S_{\mu,\nu}^{1/2} is the element of the square root inverse of the
overlap matrix S of the atomic orbitals AO. Because lowdin orbitals are
orthogonal there is no overlap between them ==> no overlap population!

I am not aware of any tool in the distribution that can do overlap
populations.

Perhaps, instead of overlap population, you may plot the atomic
projected-densities of states (using projwfc.x), and you can look how
good is the PDOS "alignment" between the atoms in question ...
Or you may plot the 2D contour plot of either charge density difference
or integrated local density of states along the bond direction.

Regards, Tone

--
Anton Kokalj
J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
(tel: +386-1-477-3523 // fax:+386-1-477-3822)

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