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

[alessandro.pecchia]

Hello Lachlan,
Yes this seems a good approach. I asked about dispersion because I don't know if the phonon tool is available in the dftb+ release yet. I think not. On the other hand my tool is based on supercell replica but if the supercell is large the computation of the Hessian becomes quite long and has to be broken for safety by selecting movable atoms. If interested let us know.With 'modes' you only get the gamma point frequencies. 
Kind regards,
Alessandro


Inviato dal mio dispositivo Samsung

-------- Messaggio originale --------
Da: Lachlan Oberg <u5805496 at anu.edu.au> 
Data: 26/07/17  03:56  (GMT+01:00) 
A: User list for DFTB+ related questions <dftb-plus-user at mailman.zfn.uni-bremen.de> 
Oggetto: Re: [DFTB-Plus-User] R:  Phonon Mode Calculations 


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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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