[Pw_forum] How to eliminate imaginary phonon frequency by adjust the crystal structure?

[Miao Gao]

Dear all:
I'm using QE-5.0.1 to calculate phonons of a hypothetical crystal.
It is known to all that an imaginary phonon frequency at non-Gamma point means
the crystal structure is not stable. My question is how to adjust the crystal
structure to eliminate the imaginary phonon frequency? My method is as follows.

(1) calculate the dynamical matrix at the q-point which has imaginary phonon frequency;
(2) diagonalize the dynamical matrix by dynmat.x;
(3) plot the imaginary phonon modes by Xcrysden to see the movements of atoms;
(4) adjust the inner-coordinations of atoms towards the direction of motion,
corresponding to the imaginary phonon modes. Relax this new structure and recalculate
phonons to see whether there is some improvement or not.

Is the procedures correct? But after relaxation, atoms do not form a stable structure and
go back to the initial positions, which has imaginary phonon frequency. Any suggestions?
Any help will be appreciated.

Best

Miao Gao

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Ph.D. student of Department of Physics,
Renmin University of China.
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