[Pw_forum] Using q2r.x - incorrect IFC symmetries

Navaneetha Krishnan navaneeth at caltech.edu
Thu Feb 19 17:55:34 CET 2015

```Thanks Paolo,

Do you think this convention is uniformly applied to any lattice with
ibrav=0? (i.e., is there a general form of the convention for lattices with
more than two atoms in the basis for eg.,)

Thanks again,
-Navaneeth

On Thu, Feb 19, 2015 at 3:35 AM, Paolo Giannozzi <paolo.giannozzi at uniud.it>
wrote:

> On Mon, 2015-02-09 at 15:45 -0800, Navaneetha Krishnan wrote:
>
> > I used ibrav=0 and my lattice vectors are [0 0.5 0.5], [0.5 0 0.5],
> > [0.5 0.5 0] in alat units. So if I understand the convention given in
> > the faq phonon section of the documentation, the first nearest
> > neighbors should have the indices [1 1 1], [5 1 1], [1 5 1] and [1 1
> > 5].
>
> The first nearest neighbors of atom 1 = (0,0,0) in cell at (0,0,0) are:
> 1.) atom 2 = (0.25, 0.25, 0.25) in cell at ( 0.0, 0.0, 0.0);
> 2.) atom 2 = (0.25, 0.25, 0.25) in cell at ( 0.0,-0.5,-0.5);
> 3.) atom 2 = (0.25, 0.25, 0.25) in cell at (-0.5, 0.0,-0.5);
> 4.) atom 2 = (0.25, 0.25, 0.25) in cell at (-0.5,-0.5, 0.0);
>
> You assume that the indices <=> lattice vector correspondence is:
>   [1 1 1] == (0, 0, 0), i.e. the two atoms are in the same cell
>   [2 1 1] == (0, 0.5, 0.5)
>   [5 1 1] == (0,-0.5,-0.5) (i.e. as above, refolded)
> etc, yielding atoms 1 2 + [1 1 1], [5 1 1], [1 5 1], [1 1 5] as
> neighbors. This makes sense (I would have made the same assumption),
> but apparently the code uses a different sign convention and thinks
> that the neighbors are either:
>   atoms 1 2 + [1 1 1], [2 1 1], [1 2 1], [1 1 2]
> or
>   atoms 2 1 + [1 1 1], [5 1 1], [1 5 1], [1 1 5]
> You may verify that these force constants have a more symmetric look
> (I did).
>
> I am not sure where such sign convention comes from, but I am quite
> sure that the interatomic force constants are used in a way that is
> consistent with the convention adopted for writing them and that
> results are correct.
>
> Paolo
>
> > From my understanding, the first nearest neighbours for Si have the
> > diagonal term in the force constant matrix +/- alpha and off-diagonal
> > term in the force constant matrix +/- beta for some alpha and beta.
> >
> >
> > Now, the [1 1 1] force constant seems to have the right symmetries :
> > atom pair : 1 2
> > 148    1   1   1  -6.40122756000E-02 11
> > 652    1   1   1  -4.29620484800E-02 12
> > 1156    1   1   1  -4.29620484800E-02 13
> > 1660    1   1   1  -4.29620484800E-02 21
> > 2164    1   1   1  -6.40122756000E-02 22
> > 2668    1   1   1  -4.29620484800E-02 23
> > 3172    1   1   1  -4.29620484800E-02 31
> > 3676    1   1   1  -4.29620484800E-02 32
> > 4180    1   1   1  -6.40122756000E-02 33
> >
> >
> > However, the [5 1 1] (and [1 5 1], [1 1 5]) force constants do not
> > have the right symmetries
> > atom pair : 1 2
> > 152    5   1   1  -3.87971851534E-03 11
> > 656    5   1   1   1.05471196450E-03 12
> > 1160    5   1   1   1.05471196450E-03 13
> > 1664    5   1   1   1.05471196450E-03 21
> > 2168    5   1   1  -3.60176689065E-04 22
> > 2672    5   1   1  -3.96403112877E-04 23
> > 3176    5   1   1   1.05471196450E-03 31
> > 3680    5   1   1  -3.96403112877E-04 32
> > 4184    5   1   1  -3.60176689065E-04 33
> >
> >
> > The first column is the line number in my file, then followed by the
> > three indices for the lattice vectors, then the force constant and
> > then finally the component (11=xx, 12=xy and so on)
> >
> >
> >
> > Does anyone have an idea why this happens?
> >
> >
> > P.S. : In the si.scf.in file, I have CELL_PARAMETERS {cubic}. I have
> > also tried with CELL_PARAMETERS {alat} and there is no difference.
> >
> >
> > Thanks,
> > Navaneeth
> > --
> > Navaneetha Krishnan Ravichandran,
> > Mechanical Engineering,
> > Caltech.
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>
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--
Navaneetha Krishnan Ravichandran,