[xcrysden] About isovalue

[Scott Beckman]

What  you do with the charge density depends on what you want from it.  
You could scale the charge by nelec, but I don't know how useful this 
is.  If you consider that some of the electrons listed in the 
pseudopotential come from the core (it is not uncommon for Ge to have 
partial core electrons) the value of nelec doesn't tell you the number 
of valence. 

Typically I use the charge density to get a qualitative picture only.  I 
like to plot (the charge density of the crystal) - (the charge density 
of isolated atoms) to allow me to see how the crystal's bonding changes 
the charge distribution. 

The qualitative assessment from this gives a hint about what the correct 
quantitative calculations should be. 



Yamit wrote:
>
> Dear Prof. Scott Beckman
>
> Thank you for the enlightenment !!
>
> About selection of isovalue:
>
> I would like to know, if I have to compare two systems (say, Si and 
> Ge), which iso-value should I select for both of them? (so that 
> comparison is valid.)
>
> Thank you once again!!!
>
> Thanks & Regards
> Amit
>
>
> On Mon, 02 Aug 2010 18:11:00 +0530 wrote
> >
>
>
> The charge is not calculated at EFermi, but it is the sum of the charge
> from the Kohn-Sham states integrated over the Brillouin zone and summed
> from the lowest state to the EFermi. 
>
>
>
> The integral over the
> charge distribution (on your XSF grid) should equal "nelec" from PWSCF. 
>
>
>
>
> The minimum and maximum value tells you the min. and max. value on the
> XSF grid.  So if you have a uniform charge concentration min=max.  For
> Si it is likely that 0.0014 will be located somewhere in an
> interstitial region and 0.87 will be located in a bond-charge. 
>
>
>
> I wouldn't say that there is any "ideal" isovalue.  I would fiddle with
> this value and look at the charge distribution.  This will allow you to
> develop qualitative picture of how your system works. 
>
>
>
>
>
>
>
>
>
>
>
> Yamit wrote:
> Hello All
>
>
>
> I am using Xcrysden to plot charge density of Silicon (example 05:
> Quantum Esspresso-4.2).
>
> *) Could anyone please tell the details of "xsf" file?
>
> ( This charge density is calculated at Fermi Energy. Please also
> correct me, if I am wrong.)
>
>
>
> *)When for Silicon: Minimum grid value :0.001411 and
>
> Maximum grid value : 0.87371
>
> What should be the ideal isovalue? Are there any rules to select the
> isovalues, from the values in *.xsf file?
>
>
>
> (For isovalue of 0.001411(min) & 0.87371(max), no charge density
> has been found. )
>
>
>
> Thank you in advance!!!
>
>
>
> Regards
>
> Amit N. Harode
>
> CRL INDIA
>
>
>
> P.S.
>
> Input file for QE's pp.x is as follows, &inputpp
>
> prefix = 'si'
>
> filplot = 'sicharge'
>
> plot_num= 0
>
> /
>
> &plot
>
> nfile = 1
>
> filepp(1) = 'sicharge'
>
> weight(1) = 1.0
>
> iflag = 3
>
> output_format = 5
>
> fileout = 'si.rho.xsf'
>
> /
>
>
>
>
>
>
>
>
>
>
>
>
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