[Pw_forum] Units of Charge Density - Again

[Lorenzo Paulatto]

Paul M. Grant ha scritto:
>
> In a perfect SI world, these units would be coulombs per cubic meter.
> In a practical, or rather intuitive, world, they would be in units of
> total fractional number of electrons per unit cell volume (in cubic
> angstroms or nanometers or bohrs (ugh)). So...what is it? And does it
> depend on the division of states between core and valence taken when
> constructing a given pseudopotential?
>
The internal unit is q/bohr^3, where q is the electron charge. It is a
bit awkward, but the correct unit of measure depend on the output file
format. but when it is exported to xcd files the cell axis are rescaled
to Angstrom, but the charge density is *not* rescaled, hence the unit of
charge density in the xcd files is q/A^3 * (A^3/bohr^3). In order to get
a consistent representation inside the xcd file you should multiply rho
times (bohr/A)^3 = 0.14818474.

In other kind of output files there should be no such problem (but I've
not checked all of them). Furthermore, xcd format is not suitable for
any subsequent postprocessing, as it is written with only 6 significant
digits.

> 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.
>
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


regards
-- 

Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
skype: paulatz
www:   http://people.sissa.it/~paulatto/