Dear Sakhrawl

Equation of state is energy as a function of volume.
Therefore the crucial information needed in the ev.x program is volume, not 
lattice parameters.
The ev.x asks structure input to be able to calculate the volume from supplied 
lattice parameters.
For example, ibrav=2, celldm(1)=a for an fcc, you input the ev.x with "a vs 
Energy" data, it calculates volume=a^3*1/4 for each data point you have 
supplied and creates a "volume vs Energy" information instead, and then uses 
this to fit the EOS..
If you input "a vs Energy" for a simple cubic instead, it would calculate 
volume as a^3.
So, in principle, it is there to make things easier for the user..

Just use "hexagonal" and input the volume instead. 
(*<http://www.timeshighereducation.co.uk/418217.article>)

emine kucukbenli, postdoc at theos, epfl, switzerland
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