[xcrysden] About isovalue

[Yamit]

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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