[Wien] plane wave cutoff for charge density

[yxl at email.jlu.edu.cn]

Dear users,
   I'm a beginner,I've some questions in pinciple of wien2k.I hope someboby will help me.
Thanks in advance.
After we do 'x dstart' in init_lapw , we should check if gmax>gmin, Why there is Gmin?
I want to know why the plane wave cutoff for the charge density needs to be at least twice that of the wavefunctions. I've searched many books ,but I can't get an answer.Please help me ,I'll be grateful for your answer.      
 hongxia