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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...).
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