[Pw_forum] Wrong effective charges in AFM insulator using "zue"

[Paolo Giannozzi]

On Jan 29, 2011, at 8:30 , xirainbow wrote:

> Why is it almost impossilbe to obtain zero for tranlational modes
> from the linear response calculation? [...]
> Does it relate with spatial discretization?

it does. Both the charge and the XC potential are calculated
in real-space on a grid. While the charge is represented exactly
on this grid (that is: all Fourier components of the charge density
are taken into account in real space), the XC potential is not,
because it is a nonlinear function (i.e. there are Fourier components
of the XC potentials that are not taken into account in real space).
This is why translational invariance is lost. Without XC potential,
the frequencies for translational modes are much closer to zero.

> Does finite displacement method (frozen phonon) can get zero?

The loss of translational invariance is real: if you calculate the  
energy
for a slightly displaced configuration, you do not get exactly  the same
energy; but it is not necessary to actually perform such a calculation,
since the result is known.

P.
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Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222