## Notes

The Raman calculation in Quantum ESPRESSO (QE) is implemented based on second order response of DFT described by Lazzeri (2003). With the `lraman`

parameter switched to `true`

, you calculate Raman tensors. Detailed example can be seen on the QE’s PHonon example 5 and example 15.

However, getting accurate Raman result from QE is very difficult. As of the year of 2019, after the release of 6.3 version of the software suite, there are still limitations.

First, the calculation is only implemented for norm-conserving pseudopotentials.

Second, the calculation for GGA-PBE is only up to second order. Lorenzo Paulatto’s presentation at ICTP conference dives into the detail and discuss the shape of the Raman spectrum. Therefore, no linewidths in phonon spectra. This incompleteness make the developer of QE to disable the calculation for GGA-PBE type exchange-correlation functions in the `PHonon/PH/phq_setup.f90`

file by default:

```
IF (dft_is_gradient().and.(lraman.or.elop)) call errore('phq_setup', &
'third order derivatives not implemented with GGA', 1)
```

But effect of such third order terms are identified to be not very significant. To force QE to perform such a calculation, one could switch off the test but perform the calculation without interruption.

```
IF (dft_is_gradient().and.(lraman.or.elop)) call errore('phq_setup', &
'third order derivatives not implemented with GGA', 0)
```

## Resources

- Vibrational Spectroscopy via Computer Simulations: Raman, Infra-Red and Related Methods by Ari P Seitsonen
- Ab Initio Simulation of Infrared and Raman Spectroscopy: Advanced Workshop on High-Performance & High-Throughput Materials Simulations using Quantum ESPRESSO by Lorenzo Paulatto
- http://www.democritos.it/pipermail/pw_forum/2008-September/010123.html
- http://www.democritos.it/pipermail/pw_forum/2010-February.txt
- http://www.democritos.it/pipermail/pw_forum/2010-November/018584.html