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this is not correct.<br>
if the code finds some symmetry, forces and stress components will
be symmetrized accordingly and vc-relax will keep it.<br>
<br>
There may be two reasons why the code does not find a symmetry:<br>
<br>
- the symmetry is not there... check your coordinates. Beware that
you need to specify ALL the atomic positions of a given structure
not just the inequivalent ones, hoping the code to fill the others.<br>
<br>
- the symmetry is there but involves a fractionary translations that
cannot be written as a multiple of the FFT grid spacing. the code
writes a message saying that the symmetry is discarded due to a
problem with fractionary translation and says what the translation
would be in crystal coordinates. one can manually set the FFT
dimensions (nr1,nr2, and/or nr3) so that the translation is in
register with the FFT grid and the code can accept it<br>
<br>
HTH<br>
<br>
stefano<br>
-<br>
Stefano de Gironcoli - SISSA and DEMOCRITOS<br>
<br>
<br>
<br>
On 07/09/2011 07:58 PM, Vic Bermudez wrote:
<blockquote
cite="mid:AKEFIBNCPJEJJMHNOPHICEGICHAA.victor.bermudez@nrl.navy.mil"
type="cite">
<pre wrap="">Hi Pascal,
If I understand correctly, you're trying to optimize the unit cell while
constraining 2 of the lattice constants to remain equal, as required by
symmetry. I don't think that QE allows this. If you do a "vc-relax", what
seems to happen is that symmetry is ignored, and all lattice constants are
varied independently. I've attached a script written by a colleague that
does repeated single-point runs for a range of lattice constants. You can
then fit points near the total-energy minimum to find the exact minimum. Of
course, this has to be done iteratively if more than one lattice constant is
being optimized.
Hope this helps.
Best Wishes,
Vic Bermudez
Victor M. Bermudez
Code 6876
U.S. Naval Research Laboratory
4555 Overlook Ave., S.W.
Washington, DC 20375-5347
</pre>
<pre wrap="">
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