CCL:G: [Summary] IR intensities from VASP
- From: Ralf Tonner <tonner#%#chemie.uni-marburg.de>
- Subject: CCL:G: [Summary] IR intensities from VASP
- Date: Tue, 05 Apr 2011 16:09:50 +0200
Sent to CCL by: Ralf Tonner [tonner ~~ chemie.uni-marburg.de]
Dear CClers,
after several very helpful answers to my above-mentioned post and
initial investigations, I want to summarize the results.
In VASP, the intensities of IR-active vibrational frequencies are not
calculated directly.
CASTEP can do this - Raman activities can be calculated as well (thanks
to Phil Hasnip).
If calculations with VASP are undertaken, two approaches can be
followed
to obtain the respective IR-intensitites:
a) change of dipole moment w.r.t. normal modes
("old" approach, also used in most molecular QC programs,
e.g. Gaussian.
See e.g. Preuss, Bechstedt Phys. Rev. B 73 2006, 155413 for an
application to VASP)
b) density functional pertubation theory (a.k.a. linear response theory).
For a), a finite difference calculation has to be carried out with
displacement of all atoms in all directions (might be reduced by
symmetry considerations). The dipole moment has to be calculated for
all
displacements.
Afterwards, the dipole moment change w.r.t the normal modes has to be
calculated which is proportional to the IR intensity of the respective
mode. Several programs are available for this task (thanks to Alfred
Gil
Arranz and Kacper Druzbicki).
This method is restricted to isolated molecules and slabs in VASP.
For b), a linear response calculation has to be carried out and the
born
effective charges have to be determined for all atoms. Afterwards, the
method published by Gianozzi&Baroni (see JCP, 100, 1994, 8537 and
Rev.
Mod. Phys. 73, 2001, 515) can be applied to derive IR intensities.
Many thanks to David Karhanek (ICIQ, Spain) for introducing me to this
method and providing a script implementing this scheme.
This latter approach is also followed e.g. by CASTEP and seems to be
more reliable and more general.
Kind regards,
Ralf.
--
Dr. Ralf Tonner <tonner() chemie.uni-marburg.de>
Philipps-Universität Marburg GERMANY
Ph +49-(0)6421-28-25418 Fax +49-(0)6421-28-25566
http://www.uni-marburg.de/fb15/ag-tonner
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