Editing Intensities.sh (section)

== DESCRIPTION ==

Although vibrational analysis is a kind of routine task, sometimes it is needed to get fast the intensities. 

Mostly we just want to "check" if our optimization finished to a reasonable minimum or that the transition state we found is a pretty saddle point we wanted to have... well, in these cases, we can be satisfied with changing the INCAR settings to be: <TT>IBRION=5</TT>, <TT>NFREE=2</TT>, <TT>POTIM="small_number"</TT> (whereas <TT>"small_number" < 0.05</TT>). Sometimes it's okay to proceed the "infrared" analysis even at lower ''k''-point grid in spite of speed.

In older VASP versions (4.6 or lower), the intensities of vibrational modes could only be calculated (with obstructions) based on the change of dipole moment for each vibration direction separately. This is the same implementation as it is being routinely used while implemented in software suites like [[GAUSSIAN]] - where the corresponding source code comes from 1970's.

As of VASP 5.* version, the DFPT linear response calculations are available. Among others, the user can obtain the matrix of ''Born effective charges'' ([http://cms.mpi.univie.ac.at/vasp/Berry_phase/node4.html BEC]), which refers to change of atoms' polarizabilities w.r.t. an external electric field. The BEC tensor is a key to calculate the vibrational intensities using the most modern method available (as far as I know!), using the formula by Gianozzi&Baroni (aplications [http://dx.doi.org/10.1063/1.466753 1] and [http://dx.doi.org/10.1103/PhysRevB.57.223 2], theory [http://dx.doi.org/10.1103/RevModPhys.73.515 3], further reading: [http://othes.univie.ac.at/10117/ David's PhD thesis], chapter 2.10.3).

For calculations with VASP DPFT formalism, include the following in the INCAR file:

<p align="left" style="width:100%">
{| border="1"
 |- 
 | <TT>IBRION = 7</TT>
 | switches on the DFPT vibrational analysis (with no symmetry constraints) ([http://cms.mpi.univie.ac.at/vasp/vasp/Optical_properties_density_functional_perturbation_theory_PT.html man])
 |- 
 | <TT>LEPSILON = .TRUE.</TT>
 | enables to calculate and prints BEC tensor ([http://cms.mpi.univie.ac.at/vasp/vasp/LEPSILON_static_dielectric_matrix_ion_clamped_piezoelectric_tensor_Born_effective_charges.html man])
 |- 
 | <TT>NSW = 1</TT>
 | reduces # of ionic steps to 1; unlike for <TT>IBRION=5</TT>, here the setting is very important
 |}
</p>

:'''Note''' In the INCAR file, please take care that you also obtain the electronic density properly converged with <TT>NELMIN=10</TT> in combination with <TT>NELMIN=120</TT> and always SWITCH OFF THE SYMMETRY with <TT>ISYM=0</TT> tag.

In case that you want to extract intensities from OUTCAR, submit the below script without any parameters (it reads the "OUTCAR" file in the current directory). The output is written into "<TT>intensities</TT>" subdirectory, the most imporant output is to be found in the file "<TT>intensities/results/results.txt</TT>", whereas "<TT>intensities/results/exact.res.txt</TT> contains intensities before normalization".

The script <TT>intensities.sh</TT> itself: