[Pw_forum] phonon calculation of alkaline metals

[Aritz Leonardo]

Hello all!

I am calculating the phonon dispersion curves of a monolayer of Li (hcp).
Crystal energies and electronic band structure of my calculation are in 
good agreement with literature but, when I calculate phonon dispersion 
curves something strange happens.

I first obtain ph frequencies in symmetry points ( gamma, K and M) and 
all of them seem to be alright except for the first branches at K and M 
which are negative.

    Gamma
         omega( 1) =      0.268417 [THz] =      -8.953502 [cm-1]
         omega( 2) =      0.260430 [THz] =      -8.687054 [cm-1]
         omega( 3) =      0.260430 [THz] =      -8.687054 [cm-1]
    K
         omega( 1) =      -2.451410 [THz] =     -81.770768 [cm-1]
         omega( 2) =       8.391945 [THz] =     279.927008 [cm-1]
         omega( 3) =       8.391945 [THz] =     279.927008 [cm-1]
    M
         omega( 1) =      -0.453435 [THz] =     -15.125059 [cm-1]
         omega( 2) =       5.259870 [THz] =     175.451548 [cm-1]
         omega( 3) =       9.222334 [THz] =     307.626006 [cm-1]

I don't understand at all what can be going on.
Thanks a lot!
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