[QUERY] Is there a way to label (IR) active phonon modes as LO and TO?

[idscum]

Hello,

This is a query rather than an error resolution.

On plotting phonon dispersions, TDEP outputs a "outfile.mode_activity.csv" file which lists the identified IR and Raman active phonon frequencies at the gamma point (sourced from activity.f90). Unfortunately, I am not very well versed with fortran to understand the calculation being done there, and therefore have the following questions: 1] Are eigenvectors being calculated (from the dynamical matrix)? (I believe the current code is already accounting for the born charges) 2] If so, can you (TDEP) use that to identify and output the respective LO (longitudinal optic) and TO (transverse optic) modes.

Alternatively, Are there any other methods to extract this information from the current version of TDEP?

Thanks in anticipation, Sreerag.

[mjv500]

Hi Seerag,

yes - the IR spectrum "activity" is proportional to the phonon mode dipole moment, which is the eigenvector multiplied by the Born charges. Eigenvectors/eigendisplacements are certainly being calculated, and if the mode dipole is finite then they are IR active. If the displacement is along q then they are LO, perpendicular TO (I guess you know this).

You can also deduce this from the eigenvector symmetries and the irreducible representations they belong to ( https://www.cryst.ehu.es/rep/sam.html).

M.

On Mon, Mar 11, 2024 at 8:05 PM Sreerag @.***> wrote:

Hello,

This is a query rather than an error resolution.

On plotting phonon dispersions, TDEP outputs a "outfile.mode_activity.csv" file which lists the identified IR and Raman active phonon frequencies at the gamma point (sourced from activity.f90). Unfortunately, I am not very well versed with fortran to understand the calculation being done there, and therefore have the following questions: 1] Are eigenvectors being calculated (from the dynamical matrix)? (I believe the current code is already accounting for the born charges) 2] If so, can you (TDEP) use that to identify and output the respective LO (longitudinal optic) and TO (transverse optic) modes.

Alternatively, Are there any other methods to extract this information from the current version of TDEP?

Thanks in anticipation, Sreerag.

— Reply to this email directly, view it on GitHub https://github.com/tdep-developers/tdep/issues/46, or unsubscribe https://github.com/notifications/unsubscribe-auth/ABWOT32L3VH6SDIPSCAAY3TYXX57HAVCNFSM6AAAAABEQ4HJNGVHI2DSMVQWIX3LMV43ASLTON2WKOZSGE3TSOJZG4ZTIOA . You are receiving this because you are subscribed to this thread.Message ID: @.***>

--

Professor Matthieu J Verstraete
Fellow, American Physical Society
Chair, Steering Committee, European Theoretical Spectroscopy Facility
www.etsf.eu
Alumnus Fellow, Young Academy of Europe  yacadeuro.org

Institute for Theoretical Physics (ITP) Department of Physics
Buys Ballot Gebouw/Building, Princetonplein 5, office
University of Utrecht,  3584 CC Utrecht
ITP Secretariat: +31 30 253 5928 E-mail: ***@***.***

Group web page (Liège): http://www.nanomat.ulg.ac.be/

Nanomat lab, Q-Mat center, Université de Liège
Département de Physique, Bat. B5a, 4/50
Allée du 6 août, 19 B-4000 Sart Tilman, Liège Belgium
Phone : +32 4 366 90 17

European Theoretical Spectroscopy Facility (ETSF)

Mail : ***@***.***
          ***@***.***
          ***@***.***

[flokno]

Hi @idscum and @mjv500 ,

yes, this is implemented, but in tdeptools. There is script that computes exactly what you want: The TO and LO projected mode-resolved Born charge. If a mode only has a TO value it's pure TO, vice versa for LO, otherwise it can be mixed. I used this here in Fig. 3 to estimate the "LO activity" of the modes.

The script is in beta status, it should be functional but it is not properly documented. If you need this feature please drop me a mail and describe your use case.