[Pw_forum] symmetries during a vc-relax
Antoine.JAY at isae-supaero.fr
Mon Oct 2 16:25:37 CEST 2017
Dear devellopers, dear all,
I would like to perform a vc-relax that concerves the point group of each atom and the symmetry of the cell.
I know it is possible to impose symmetries on the BZ's k-points or to fix some atoms coordinates, but this does not give the expected result.
For example,Â in a trigonal cell, ibrav 5 imposes the symmetries:
v1 = a(tx,-ty,tz)
v2 = a(0,2ty,tz)
v3 = a(-tx,-ty,tz)
but these symmetries seem to be broken by the relaxation,
so that at the end of the relaxation, v2(1) is not exactelly null and v1(1) is not exactelly -v3(1),
or for the atomic positions, a coordinate (x, x, x) alat becomes (x, 0.99 x, 1.001x ).
?? Is it possible to impose that these constraints on the unit-cell and on the atomic positions stay fix during the relaxation process ??
This is a major issue for the use of these relaxed values in a second step for phonons for example, because it will then not detect the symmetries.
I think it could be a good idea to add into the vc-relax output file a lign that gives the corresponding celldm(i) and not only the CELL_PARAMETERS.
In fact, in the scf before the phonon use, we would like to have ibrav =/0 and celldm in state of ibrav 0 and quasi symmetric CELL_PARAMETERS...
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