# [Pw_forum] Using q2r.x - incorrect IFC symmetries

Paolo Giannozzi paolo.giannozzi at uniud.it
Sat Feb 21 10:00:16 CET 2015

```On Thu, 2015-02-19 at 08:55 -0800, Navaneetha Krishnan wrote:

> 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.,)

I don't now, but I don't think there is anything specific to two-atom
systems in the code

Paolo

>
>
> 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,
>         > Graduate Student,
>         > Mechanical Engineering,
>         > Caltech.
>
>         > _______________________________________________
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>
>
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>
>
>
> --
> Navaneetha Krishnan Ravichandran,
> Graduate Student,
> Mechanical Engineering,
> Caltech.
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--
Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222

```

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