Dear Giaocomo,
Just to follow up on Tatsuhiko comments, Vibra uses a real space
approach to calculate the interatomic force constants (moving each
unique atom in the central unit cell of in the supercell). This
approach is fine for calculating the analytic terms of the interatomic
force constants. However, for polar materials, you also get a
non-analytic terms at q=0 that depends on the Born effective charges of
the ions and the dielectric constant for the system. This term gives
you the LO-TO splitting in the optical branches. This term can not be
calculated using the real space approach implemented in Vibra.
To calculate this, you could determine the Born effective charges using
a different technique (like density functional perturbation theory - a
linear response approach) in a code like Quantum Espresso or Abinit and
add this to the force constants calculated with Siesta. From the
vibrasource code, it looks like it reads in the Born charges for the IR
intensity, althoughI haven't used this functionality. As a check of your
results, you could also use Quantum Espresso or Abinit to calculate the
full phonon dispersion using density functional perturbation theory.
You can find more information on density functional perturbation theory,
Born charges, and LO-TO splitting in the following review article:
"Phonons and related crystal properties from density-functional
perturbation theory", Rev. of Modern Physics, 73, 515, 2001
Best regards,
Derek
################################
Derek Stewart, Ph. D.
Senior Research Associate
http://sites.google.com/site/dft4nano/
250 Duffield Hall
Cornell Nanoscale Facility (CNF)
Ithaca, NY 14853
stewart (at) cnf.cornell.edu
(607) 255-2856
On 10/25/2012 3:22 AM, Tatsuhiko Ohto wrote:
Dear Giacomo,
I found that the Vibra accompanied with SIESTA-3.1 can calculate IR
intensities.
Intensities .true.
in the input of Vibra with .FC and .BC.
Cheers,
Tatsuhiko
----- Original Message ----- From: "Giacomo Giorgi" <[email protected]>
To: <[email protected]>
Sent: Friday, October 05, 2012 12:57 PM
Subject: Re: [SIESTA-L] LO-TO splitting and IR spectrum
Dear All,
actually I realized that about other 20 people experienced my
situation without getting the code for obtaining the IR spectrum....
I wonder if, without such code, it is still possible to calculate
LO-TO splitting simply from .FC and .BC and possibly the IR spectrum
too.
Excuse me I repeated the question but I really would need an answer.
With very best regards,
Giacomo
On Tue, 2 Oct 2012, Giacomo Giorgi wrote:
Dear Users,
I got my .FC and .BC files from the .FC calculation.
I have written to Tom Archer in order to get the tool for
calculating the IR spectrum (as written in the Vibra manual),
without anyway receiving any reply.
Can anyone share it with me?
Thanks in advance!
Best,
Giacomo
P.S.: From the same FC calculation how to calculate the LO-TO
splitting?
--
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"Oltre le illusioni di Timbuctu'
e le gambe lunghe di Babalu' c'era questa strada...
Questa strada zitta che vola via come una farfalla,
una nostalgia, nostalgia al gusto di curacao...
Forse un giorno meglio mi spieghero'..."
(Paolo Conte, "Hemingway")
