[Wien] position optimization, volume optimization?

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Dear wien user. 

I'm beginner of wien2k and plan to use it for the ELNES study of HfO2.

To simulate the variation of ELNES peak in the case of oxygen vacancy, I made the supercell of monoclinic HfO2 and removed one Oxygen site. 

What I know is that after I removed the oxygen from supercell, the relaxation of the super lattice is necessary. 
But I'm not sure how far I must go.  So just the position optimization is enough for simulation. or I have to do volume optimization?  

My best

mino yang from korea.