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[corsin.battaglia@freesurf.ch]

Dear abinit users

I am still calculating phonons for the metallic compound NbTe2.

In my initial unrelaxed calculation at the experimental crystal structure, 
the phonon bandstructure exhibited imaginary frequencies discussed several 
times on the ML. After relaxation of the structure (maxfor=0.022 eV/A), these 
imaginary phonon modes disappeared.

In an article by Ph. Ghosez et al, which I found at
www.gl.ciw.edu/~cohen/meetings/ferro2000/proceedings/ghosez.pdf, phonon
dispersion relations for ABO3 compounds are calculated at the experimental 
lattice constants, although this corresponds to the effect of an external 
pressure of 8.7 GPa. In Ph. Ghosez et al., PRB 60, 836 (1999) calculations 
for BaTiO3 are also carried out at the experimental volume. The resulting LDA 
phonon dispersion relations exhibit imaginary frequencies.

My calculation at the experimental crystal structure of NbTe2 corresponds to 
a pressure of 4 GPa. In fact, my results are obtained for an idealized 
hexagonal crystal structure, whereas the real laboratory crystals exhibit a 
monoclinically distorted version of this idealized structure. Instabilities 
in the phonon bandstructure are found at q-vectors, coinciding approximately 
with the experimentally observed modulation of the monoclinic structure. This 
has also been observed by X. Gonze et al. in 
www.gl.ciw.edu/~cohen/meetings/ferro2000/proceedings/gonze.pdf for 
two-dimensional PbO layers.

Under which conditions, one can work at unrelaxed structure parameters? My 
current knowledge suggests that the interesting imaginary phonon branches 
necessarily disappear for a LDA relaxed structure (harmonic approximation 
valid). To which extend the interpolation performed by anaddb is responsible 
for imaginary modes (in the case of PbO, increasing the q-point grid results 
in a weaker instability, direct calculation yields no instability)? Do the
imaginary modes become stabilized (real valued), when anharmonic terms are 
included in the calculation? Is it justified to make predictions concerning 
possible phase transitions based on these instabilities?

Thanks in advance and best regards

Corsin



P.S.: I have troubles to converge my calculations for larger values of ecut, 
whereas for small values the calculation converges (details will follow...).