[Pw_forum] matdyn error

[Mikiyas Tsegaye]

Dear Rakshit,

I'm just copy pasting from a previous answer in the forum to the same  
question: it might help, it might not.

Hello,
   Assuming that there are no problems of numerical inaccuracy or  
inadequate convergence
thresholds, negative frequencies at gamma point in a geometry-relaxed  
system usually mean that
(within the system and the calculation method used) there is some  
distortion (decreasing the
symmetry of the system) which leads to lower energy, i.e. you are at a  
saddle point in the energy
hypersurface. You must move some atom out of the initial symmetry and  
optimize geometry
again starting from that modified situation. If you can obtain the  
vector coordinates for the normal
mode of vibration corresponding to the negative frequency, a  
displacement of the involved atoms,
proportional to that normal mode, by e.g. a maximum of 0.2 angstrom  
will give to you a starting
displaced geometry closer to the true energy minimum. Sometimes you  
need to do this several
times if not all of the negative frequencies disappear after the first  
attempt.
If, after obtaining a new geometry with no negative frequencies at the  
gamma point, you still have
negative frequencies at other BZ points, this means that your system  
is unstable against some
atomic displacements that multiply some of the primitive cell vectors  
by e.g. 2 or 3, leading to a
supercell. Curing this situation may be more tedious.
Good luck,
Jose C. Conesa
Instituto de Catalisis y Petroleoquimica, CSIC
Campus de Cantoblanco
28049 Madrid  -  Spain
Phone Nr. 34-91-5854766
Fax Nr. 34-91-5854760

>
>
> Message: 3
> Date: Fri, 14 Aug 2009 11:25:23 +0530
> From: Bipul Rakshit <bipulrr at gmail.com>
> Subject: [Pw_forum] matdyn error
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID:
> 	<3a749910908132255x13311317j4eb55858c46affe0 at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Dear pwscf users,
>
> I am doing Gamma point phonon analysis for LaMnO3, after running  
> matdyn.x i
> am getting the following, error.
>
> Norm of the difference between old and new effective charges:
> 0.00000000000000000000
> Norm of the difference between old and new force-constants:
> 0.20461738165775616993
>  0.833333333333333
>
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 
> %%%%%%%%%
>     from frc_blk : error #         1
>     wrong total_weight
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 
> %%%%%%%%%
>
>     stopping ...
> 2
>
> Since at gamma point i am getting some -ve freq. the sum rule i want  
> to
> apply. And to see the final frequencies after applying sum rule.
>
> Kindly guide me and suggest for the possible reason for this error.
> -- 
> Dr. Bipul Rakshit
> Research Associate,
> S N Bose Centre for Basic Sciences,
> Salt Lake,
> Kolkata 700 098
> India
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