vasp_raman.py

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

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
• 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.

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

0.6 (will be released soon)

• 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

Contributors

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