[Pw_forum] doubt in ph.x and matdyn.x

[Bipul Rakshit]

Dear users,
I am doing the phonon calculation of Graphene sheet.
In the phonon input file "graph.ph.in" i am using nq(i)= 8x8x1, which gives
the following q-points

     *Dynamical matrices for ( 8, 8, 1,)  uniform grid of q-points
     (  10q-points):
       N         xq(1)         xq(2)         xq(3)
       1   0.000000000   0.000000000   0.000000000
       2   0.000000000   0.144337567   0.000000000
       3   0.000000000   0.288675135   0.000000000
       4   0.000000000   0.433012702   0.000000000
       5   0.000000000  -0.577350269   0.000000000
       6   0.125000000   0.216506351   0.000000000
       7   0.125000000   0.360843918   0.000000000
       8   0.125000000   0.505181486   0.000000000
       9   0.250000000   0.433012702   0.000000000
      10   0.250000000   0.577350269   0.000000000*

and in the matdyn1.in file (input file for matdyn.x)  i used
*G 0.0 0.0 0.0
K 0.3333 0.57735  0.0000
M 0.0000 0.57735  0.0000
G 0.0000 0.00000  0.0000*

high symmetry points, along with other intermediate point.

So my question is, when i plot the frequencies (*.freq file), given after
running matdyn.x file, using the above high symmetry points, the dispersion
is all positive (the frequencies are all positive)

But when I am doing single *q=0.3333 0.57735  0.0000* point calculation in
my graph.ph.in file. For this perticular q-point the frequencies are -ve.

Can anybody tell me the reason for this, or I am doing some error?

-- 
Bipul Rakshit
Research Fellow
S N Bose Centre for Basic Sciences,
Salt Lake,
Kolkata 700 098
India
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