[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.
>
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://pwscf.org/mailman/listinfo/pw_forum
>
>
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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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