[Pw_forum] How to improve the precision of phonon calculation?

[Uri Argaman]

Dear Yu Hailin

In questions like this you should show the SCF input that you use. The
negative frequencies show that the crystal is unstable or your calculations
are not converged. It can be the numerical parameters of the SCF
calculation or the numerical parameters of the phonon calculation. As far
as I know, the only numerical parameter used in the phonon calculation is
tr2_ph.

If your calculation is converged than the negative (imaginary) frequencies
show that the structure is unstable. You must do relaxation (probably to
zero pressure) before the SCF to ensure that the structure is on the
minimum of the energy surface (at list local minimum).

Best regards

Uri Argaman

Ben-Gurion University

Israel
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