[Pw_forum] Wrong phonon frequencies when using nosym=false

[Tao Sun]

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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