[Pw_forum] Relax in primitive cell or conventional cell

[Peng Chen]

Dear All,

I am trying to optimize the structure based on the experimental crystal
structure using 'relax' task.  Shall I use primitive cell or conventional
unit cell to do relax? The primitive cell job (26 atoms) is done and
conventional cell job (52 atoms) is still in progress. I used the same
parameters for both jobs and it looks like the results will be different.

Another quick question: the calculated energy for primitive cell (26 atoms,
3 types) is converged into 0.001 Ry. Is this energy difference (0.001 Ry)
for 26 atoms? In this case, can we say that total energy is converged
within 0.001/26 Ry/atom?  Some workers wrote in their paper that the energy
is converged within 0.5 eV/atom. It is a little weird expression to me
because there are several different types of atoms.

-- 
  Best Regards.
        Peng
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