[QE-developers] Raman spectra with LDA(GGA)+U

[Lorenzo Bastonero]

Dear Paul,

Just to clarify this point:

> While Vasp can carry out Raman calculations using phon3py […]

Phono3py can interface Quantum ESPRESSO as well, since this code simply needs forces of supercells with finite displacements to evaluate third order force constants. (See also its doc <https://phonopy.github.io/phono3py/>). These are needed in case you would like to have the phonon line widths at different temperatures.

While the computation of phonon line widths is very onerous indeed, it is usually not needed for a comparison with experiments. Moreover, you don’t need them for the computation of the Raman intensities. For this, you need to evaluate susceptibility derivatives in respect to atomic displacements, often called Raman tensors. 

These tensors are third order derivatives of the total energy in respect to 2 electric fields and 1 atomic displacement. Thus, there are multiple way to compute them: 2n+1 theorem, derivatives of the charge density and finite differences.

Usually, the implementation of the former 2 are much more efficient, but also very involved in the implementation. This is the reason why in plane-waves codes their implementations are limited to LDA(+U). In Quantum ESPRESSO we are currently limited to LDA and NC pseudos.

To overcome this limitation on the functional (and pseudo), finite differences can still be employed, using e.g. the electric enthalpy approach (see Umari and Pasquarello, Diamond & Related Materials 14 (2005) 1255–1261). There are already some routines in Quantum ESPRESSO, in the PHonon/FD folder, to carry out these calculations semi-automatically. 

Since the approach requires many calculations (and there are some pitfalls as well), one year ago I and my supervisor (Prof. Nicola Marzari) decided to implement in the AiiDA <https://www.aiida.net/> framework a workflow for doing all these calculations automatically. The package is a python plugin which extends the functionalities of AiiDA and its aiida-quantuemespresso plugin; the latter already manages many of the usual QE executables). 

This plugin will be made open source very soon at the link https://github.com/bastonero/aiida-vibroscopy <https://github.com/bastonero/aiida-vibroscopy>, and also on the AiiDA registry as aiida-vibroscopy.

I hope this is and will be useful.

Best regards,
Lorenzo Bastonero

********************************
Lorenzo Bastonero

PhD Student
U Bremen Excellence Chair,
Bremen Center for Computational Materials Science,
and MAPEX Center for Materials and Processes

University of Bremen
Faculty of Production Engineering
TAB-Building, Room 3.32
Am Fallturm 1
28359 Bremen, DE

http://www.hmi.uni-bremen.de/

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