[Pw_forum] Units of Charge Density - Again

[Paul M. Grant]

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

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.

Paul M. Grant, PhD
Principal, W2AGZ Technologies
Visiting Scholar, Applied Physics, Stanford (2005-2008)
EPRI Science Fellow (Retired)
IBM Research Staff Member Emeritus
w2agz at pacbell.net
http://www.w2agz.com <http://www.w2agz.com/> 
 
 


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