[Pw_forum] Negative freuqencies for extrapolating points

[Elio Physics]

Dear Professor Paulo and Fransesco,


thanks for your help and suggestions



________________________________
From: pw_forum-bounces at pwscf.org <pw_forum-bounces at pwscf.org> on behalf of Paolo Giannozzi <p.giannozzi at gmail.com>
Sent: Thursday, August 25, 2016 12:41 PM
To: PWSCF Forum
Subject: Re: [Pw_forum] Negative freuqencies for extrapolating points

I am aware of other cases in which acoustic phonon frequencies close to q=0 behave in a funny way. It happens in very small regions and resists to standard attempts (increasing convergence etc.) to get rid of it. No explanation and no fix available for now.

Paolo

On Thu, Aug 25, 2016 at 4:03 PM, Elio Physics <Elio-Physics at live.com<mailto:Elio-Physics at live.com>> wrote:

Dear all,


I have calculated the frequencies for a structure. All frequencies were positive except for a few "extrapolated" points very close to the Gamma point which were negative in the range of -1.45 cm^-1. Due to this the dispersion curve near the Gamma point looked a bit funny (but looks pretty good elsewhere).  I tried the calculations with 2 different phonon grids. Any reason why this is happening? Do i have to lower  tr2_ph more (it is already 10^(-19)!)


Any help would be appreciated.


Elio

Fed Uni of Rondonia

Brazil


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--
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222

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