[DFTB-Plus-User] R: Phonon Mode Calculations

[Lachlan Oberg]

Hello Alessandro,


Thank you for your reply. Yes, we are looking to produce phonon dispersion relations in periodic structures. Afterwards, we were looking to produce phonon dispersion relations in nanowire structures (approximately 50x50 Angstrom). Is this possible if we were to obtain accurate skf files (i.e., if we first optimised to known phonon dispersion relations, say, in the bulk material)?


Kind regards

Lachlan Oberg

________________________________
From: DFTB-Plus-User <dftb-plus-user-bounces at mailman.zfn.uni-bremen.de> on behalf of alessandro.pecchia <alessandro.pecchia at ismn.cnr.it>
Sent: Wednesday, July 26, 2017 1:18:07 AM
To: User list for DFTB+ related questions
Subject: [DFTB-Plus-User] R: Phonon Mode Calculations

Dear Lachlan,

you need sk-files with repulsive part included otherwise you do not get the right forces. Then in the input you set SecondDerivatives as diver. The Hessian is currently computed with finite differences.
The 'modes' code uses a LAPACK eigensolver for real symmetric matrices, if I remember correctly.
Good relaxed structures and electronic bandstructures do not necessarily implay accurate vibrations as that's related to the curvature around the minimum, harder to get in many cases. It depends if you can live with about 10-20% accurate frequencies. Do you need phonon dispersions ?

Kind Regards,

Alessandro Pecchia


Inviato dal mio dispositivo Samsung


-------- Messaggio originale --------
Da: Lachlan Oberg <u5805496 at anu.edu.au>
Data: 24/07/17 07:31 (GMT+01:00)
A: dftb-plus-user at mailman.zfn.uni-bremen.de
Oggetto: [DFTB-Plus-User] Phonon Mode Calculations


Hello,


I have two questions regarding the MODES algorithm used to calculate phonon modes. Firstly, what method does it use to calculate the eigenmodes given the Hessian? Secondly, what part of the .skf files is required to determine the Hessian? Are the integral tables sufficient, or do I explicitly require .skf files whose geometry can be optimized (i.e., they contain the repulsive spline interaction)? For example, does a faithful reproduction of the conduction/valence bands necessarily mean a faithful reproduction of the modes?


Kind regards

Lachlan Oberg
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