[Pw_forum] Phonon calculation, but inexact atomic coordinates

[Stefano de Gironcoli]

DFPT compute the phonon frequency from the curfature of the total energy

with respect to atomic positions. The fact that the atomic configuration
is
not exactly the equilibrium one will modify the curvature but should not

induce additional errors.

How much the frequency are modified ? I think that one can estimate the
effect
from gruneisen parameters which are tipically of order unit... that is a
change (or error)
in volume of 1% will induce a change in frequency of 1% ... or
converting this
number in terms of bond-lengths I would say that a change in bonding
distance
by 1% would give a produce a change in the frequency by 3%.

Therefore, provided the relaxation run determine the equilibrium
structural properties
within a fraction of % I would be rather confident on the resulting
phonon frequencies.

regards

Stefano de Gironcoli

dsa fda wrote:

> Dear all,
>
> I'm trying to apply phonon calculation for a difficult
> case, which are carbon nanotubes. Before I begin,
> there is a question on the accuracy of the expected
> result:
>
> To my understanding, when applying the linear response
> method to calculate purtabations like phonon, it is
> crucial that the EXACT atomic coordinates on
> unperturbed geometries are determined first by
> structural relaxation run. Am I right?
>
> But to my experiences, for systems like carbon
> nanotubes(several tens of atoms in unit cell), the
> accuracy of atomic coordinates can be determined no
> better than about 0.01 Angstrom by structural
> relaxation run. Is it justified to make such an
> estimation?
>
> So, suppose we already have an error bar of 0.01
> Angstrom on atomic positions in UNPERTURBED geometries
> of carbon nanotubes, then how large will the error be
> on the final phonon frequencies by PWSCF, or any
> other DFPT calculations, caused by such inaccuracy on
> atomic coordinates?
>
> P.S. for high symmetry crystals such errors do not
> happend, because atoms there all have fractional
> coordinates.
>
> Any reply is appreciated.
>
> Linhui
>
> __________________________________________________
> Do you Yahoo!?
> Yahoo! Mail Plus - Powerful. Affordable. Sign up now.
> http://mailplus.yahoo.com
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum