[Pw_forum] Phonon calculation, but inexact atomic coordinates

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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



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