[Pw_forum] vc-relax with atomic position fixed.

[Stefano de Gironcoli]

>
>
>   Can one use vc-relax option to optimize the lattice
>paramters for a fixed volume(for example optimizing "a" and "c" paramters
>for a given fixed volume) with the atomic position fixed ( b'cause they
>are already at the high symmetry positions and therefore donot need any
>relaxation)?
>
>  
>
with the variable cell relaxation option you can look for combined 
relaxation of atomic positions and cell at a fixed pressure (not 
volume... the two things are of course related)

If the positions are already correct (for symmetry reasons) then the 
internal forces acting on them should vanish and they will not 
participate in the relaxation.

stefano