CCL:G: [Summary] IR intensities from VASP



 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.
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 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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