[Pw_forum] Wrong phonon frequencies when using nosym=false

[Paolo Giannozzi]

It looks like a problem with a symmetry having an
associated fractional translation. It would be
interesting to know whether this problem is still
present in the svn version, and if not, a smaller
test showing the same problem

P.
On Mon, 2013-11-25 at 23:05 +0800, Tao Sun wrote:
> Dear users,
> 
> 
> When we calculate the phonon spectrum of a rather obscure mineral,
> K2Si4O9, at high pressures, we encounter a strange problem.
> 
> 
> The unit cell contains 60 atoms, for which the code found two Sym.
> ops. One is the identical operator, the other is 180 deg rotation -
> cart. axis [0,0,1] plus (0,0,0.5) fractional translation.  Using the
> default 'nosym = .false.' in the scf input, we found large imaginary
> frequencies at q=(0,0,1/2),
>  
>      omega( 1) =     -89.679305 [THz] =   -2991.379628 [cm-1]
>      omega( 2) =     -89.679305 [THz] =   -2991.379628 [cm-1]
>      omega( 3) =     -88.598203 [THz] =   -2955.317953 [cm-1]
>      omega( 4) =     -88.598203 [THz] =   -2955.317953 [cm-1]
>      omega( 5) =       2.408244 [THz] =      80.330387 [cm-1]
>      omega( 6) =       2.408244 [THz] =      80.330387 [cm-1]
>      omega( 7) =       2.465075 [THz] =      82.226043 [cm-1]
>      omega( 8) =       2.465075 [THz] =      82.226043 [cm-1] ...
> 
> 
> Searching more stable structures along the presumed 'unstable' modes
> was in vain, which led us to suspect the above imaginary frequencies
> are artifacts.  Indeed, by setting the 'nosym = .true.',  we found
> 
>      omega( 1) =       2.289272 [THz] =      76.361882 [cm-1]
>      omega( 2) =       2.289349 [THz] =      76.364467 [cm-1]
>      omega( 3) =       2.373282 [THz] =      79.164179 [cm-1]
>      omega( 4) =       2.373320 [THz] =      79.165434 [cm-1]
>      omega( 5) =       2.668095 [THz] =      88.998078 [cm-1]
>      omega( 6) =       2.668166 [THz] =      89.000448 [cm-1]
>      omega( 7) =       2.867070 [THz] =      95.635151 [cm-1]
>      omega( 8) =       2.867215 [THz] =      95.639985 [cm-1]...
> 
> 
> This problem is independent of the pseudopotentials used, and appears
> in pwscf 4.3.2 as well as the patched 5.0.3.
> 
> 
> The relevant input and output files are attached below.  To speed up
> reproducing the error, the k-point mesh was trimmed to gamma point
> only.  
> 
> 
> Thank you.
> 
> Tao Sun
> 
> College of Earth Sciences
> 
> University of Chinese Academy of Sciences
> 
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222