) 
with all other modes virtually unchanged, you can trust your results.
``The problem is [...] in the fact that the XC 
energy is computed in real space on a discrete grid and hence the
total energy is invariant (...) only for translation in the FFT
grid. Increasing the charge density cutoff increases the grid density
thus making the integral more exact thus reducing the problem,
unfortunately rather slowly...This problem is usually more severe for
GGA  than with LDA because the GGA functionals have functional forms
that vary more strongly with the position; particularly so for
isolated molecules or system with significant portions of ``vacuum''
because in the exponential tail of the charge density a) the finite
cutoff  (hence there is an effect due to cutoff) induces oscillations
in rho and b) the reduced gradient is diverging.''(info by Stefano de
Gironcoli, June 2008) 
 
Possible reasons
- if this happens only for acoustic modes at  = 0
 that should
  have = 0
 that should
  have = 0
: Acoustic Sum Rule violation, see the item before
  this one. = 0
: Acoustic Sum Rule violation, see the item before
  this one.
- wrong data file read.
- wrong atomic masses given in input will yield wrong frequencies
  (but the content of file fildyn should be valid, since the force
  constants, not the dynamical matrix, are written to file). 
- convergence threshold for either SCF (conv_thr) or phonon
  calculation (tr2_ph) too large: try to reduce them. 
- maybe your system does have negative or strange phonon
  frequencies, with the approximations you used. A negative frequency
  signals a mechanical instability of the chosen structure. Check that
  the structure is reasonable, and check the following parameters: 
- The cutoff for wavefunctions, ecutwfc
- For USPP and PAW: the cutoff for the charge density, ecutrho
- The k-point grid, especially for metallic systems.
 
Note that ``negative'' frequencies are actually imaginary: the negative
sign flags eigenvalues of the dynamical matrix for which < 0
.
 < 0
. 
Verify the q-vector for which you are calculating phonons. In order to
check whether a symmetry operation belongs to the small group of  ,
the code compares
,
the code compares  and the rotated
 and the rotated  , with an acceptance tolerance of  
10-5
 (set in routine PW/eqvect.f90). You may run into trouble if
your q-vector differs from a high-symmetry point by an amount in that
order of magnitude.
, with an acceptance tolerance of  
10-5
 (set in routine PW/eqvect.f90). You may run into trouble if
your q-vector differs from a high-symmetry point by an amount in that
order of magnitude.
Subsections
 
 
 
 
 
 
 
  
 Next: A. Appendix: Electron-phonon coefficients
 Up: User's Guide for the
 Previous: 5 Parallelism
     Contents 
Layla Martin-Samos Colomer
2012-11-21