[Pw_forum] Negative phonon frequencies in rotational modes

[neclaoz]

Hello everyone,

I am making a phonon calculation using ph.x. I have a 2D structure with 5
atoms in the unitcell. I use perodic bounday conditions. My phonon results
are all perfectly fine except for one (almost) horizontal band apperaing in
the negative region.

In the quantum espresso web site I read a notice saying that negative
frequencies can arrive for rotational modes of a molecule. When I look at
the details of the negative band, it is at the irreducible representation
corresponding to  A mode, which suggests that it is a rotational mode (I
think?)

My question is, what is the reason of such an artifact? My structure is not
a single molecule but a supercell, so does it also apply to my calculation?
Or should I look for something else?

Any kind of help is appreciated.

Veli
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