[Pw_forum] Variable cell

[Eyvaz Isaev]

Dear developers and users,

I am quite agree there is no time to submit any question, it is time to be prepared to incoming X-Mas.

Nevertheless, sorry, one question. I am doing variable-cell calculations by means of damp-w option. 
Initial choice for the lattice is ibrav=12, i.e. simple monoclinic. As far as I know the (Wentzcovitch) method should not change the cell shape (i.e. basis vectors will not change their symmetry). As it is mentioned in the original paper "... eliminate symmetry breaking"

But it seems to be not the case for a mononclinic cell, as initial basis vectors are:

a1 = 1     0   0
a2 = 0.0340004 0.999671 0
a3 = 0     0   0.999225

then right after the first step (i.e. damp-w starts) these vectors become

a1 = 0.999348118 -0.000624697 0.
a2 = 0.029359585 0.999030676  0
a3 = 0       0    0.998400159

I expected basis vectors will keep the original symmetry, though a1 seems to be changed. For tetragonal symmetry there is no such kind problem.

I have just tested it using v.4.0.4, the same result.

It would be very nice if someone can explain me why this happens for a monoclinic cell. May be there is some misunderstanding of mine side.

Bests,
Eyvaz.

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Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com