[Pw_forum] Units of Charge Density - Again

[Stefano Baroni]

On Sep 15, 2008, at 5:00 AM, Paul M. Grant wrote:

> There have been a number of posts to pw_forum regarding the units  
> employed for charge density throughout PWscf package and XcrySDen.   
> This is an important issue, and I still find the situation and  
> answers provided in the forum archives ambiguous.  Before slogging  
> through relevant source codes, and testing by writing time-consuming  
> homemade examples, I’m hoping someone in the community has preceded  
> me.
>
> 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?
>
Paul: when I was familiar with the code (well, it is some time  
ago ...) it used to be (bohrs)^(-3). i.e. "densities" used to be  
NUMBER densities (length^(-3)), in the units used by the code. the  
only ambiguity about units is between the "pwscf" and "CP" branches of  
the distribution. The first uses Rydberg au (e^2/2=2m=hbar=1), the  
second Hartree au (e=m=hbar=1). in both sets of units, the unit length  
is the bohr, hence no ambiguity here.

> And does it depend on the division of states between core and  
> valence taken when constructing a given pseudopotential?
>

sure it does. the integral ofer the unit (super-) cell of the density  
is the number ov valence electrons.
> 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.
>
no hint here

cheers - SB



---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
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