vasp_raman.py

Raman off-resonant activity calculator using VASP as a back-end.

TOC

Theory
Installation
Global variables
Examples
Changelog
How to cite
Contributors

Theory

In order to calculate off-resonance Raman activity of a mode, one needs to compute the derivative of the polarizability (or macroscopic dielectric tensor) with respect to that normal mode coordinate: dP/dQ (or de/dQ).
Thus, two ingredients are required:

1. Phonons at Γ-point
2. Macroscopic dielectric tensor

Phonons at Γ-point

In VASP, phonons at Γ-point can be computed using either:

• finite displacements: IBRION=5 or IBRION=6; or
• density functional perturbation theory (DFPT): IBRION=7 or IBRION=8.

Only finite displacements are available when hybrid functional is employed.

Macroscopic dielectric tensor

In VASP, macroscopic dielectric tensor can be computed using either:

• DFPT: LEPSILON=.TRUE.
• or from frequency dependent dielectric matrix calculation: LOPTICS=.TRUE..

In the latter case, hybrids functionals could be employed.
For a more formal description of the method see D. Porezag, M.R. Pederson, PRB, 54, 7830 (1996).

Installation

Python >= 2.6 is required. Just copy vasp_raman.py in the $PATH and run! No external dependencies.

Global variables

vasp_raman.py requires two environmental variables to be set:

• VASP_RAMAN_PARAMS is defined as FIRST-MODE_LAST-MODE_NDERIV_STEPSIZE where:

  • FIRST_MODE - integer, first mode for which derivative of the polarizability is computed
  • LAST-MODE - integer, last mode for which derivative of the polarizability is computed
  • NDERIV - integer, scheme for finite difference, currently only value 2 is supported
  • STEPSIZE - float, step-size for finite difference, in Angstroms

  Example: VASP_RAMAN_PARAMS=01_10_2_0.01

• VASP_RAMAN_RUN the command to execute VASP (can contain MPI call):
  Example: VASP_RAMAN_RUN='aprun -B /u/afonari/vasp.5.3.2/vasp.5.3/vasp &> job.out'

Both variables should be exported (in Bash language) before running vasp_raman.py.

An example of PBS script:

#!/bin/bash
#PBS -l select=1:ncpus=32:mpiprocs=32
#PBS -l walltime=01:00:00
#PBS -q debug
#PBS -j oe
#PBS -N Example
#PBS -V

cd $PBS_O_WORKDIR

ulimit -s unlimited  # remove limit on stack size

export VASP_RAMAN_RUN='aprun -B /u/afonari/vasp.5.3.2/vasp.5.3/vasp &> job.out'
export VASP_RAMAN_PARAMS='01_10_2_0.01'

python27 vasp_raman.py > vasp_raman.out

An example of bash script (in case no scheduler is installed):

#!/bin/bash

# suggested by Ricardo Faccio, Universidad de la República, Montevideo, Uruguay

# OpenMP variables
#export OMP_NUM_THREADS=1
#export MKL_NUM_THREADS=1

# vasp_raman.py variables
export VASP_RAMAN_RUN='mpirun -np 4 vasp5.3.5_par'
export VASP_RAMAN_PARAMS='01_06_2_0.01'

python /home/user/bin/vasp_raman.py > vasp_raman.out

Examples

• Raman activity spectrum for Si using VASP
• Raman activity spectrum for Si using VTST tools
• Raman activity spectrum for cyclopentadiene using VASP
• Raman activity spectrum for Si using VTST tools and PW91 functional

Changelog

Current

• Switched Python 3

0.6.0

• ADDED: ability to use phonons obtained from the vtst tools
• FIX: cleaned POSCAR parsing code

0.5.1

• FIX: contributors and version are now in the output
• FIX: Cyclopentadiene example is now fully consistent with the version

0.5

• Basic working functionality

How to cite

Use Bibtext or RIS file for citation.

Contributors

Alexandr Fonari (Georgia Tech, PIs: J.-L. Bredas/V. Coropceanu): Email
Shannon Stauffer (UT Austin, PI: G. Henkelman): Email.