[Pw_forum] phonon calc. for fcc at X

[Stefano de Gironcoli]

If I remember correctly there used to be a term in the phonon 
calculation in which the perturbing potential was written as DeltaV(r) = 
exp(+iqr)*\DeltaVperiodic(r) and \DeltaVperiodic was expanded up to
the charge density cutoff ... this term was therefore slightly different
for two q wavevectors that would otherwise be equivalent. This never
gave any noticeable difference in real calculations and I would be 
surprised if this would be the cause of your results but a way to check
this is to increase the charge density cutoff and see what happens as
in this case the problem should disappear when the potential description
become complete.