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Calculation of dynamical matrix from single q-point after mrgddb

Posted: Fri Feb 19, 2021 4:59 am
by jackbaker
Hi everyone.

I hope this will be a simple question but I cannot find anything on the forum about it.

How does one calculate all of the mode frequencies/the dynamical matrix (with either anaddb or abinit) from a single q-vector when you did the calculation in parts? I.e, the cell was so large that a single q-point DFPT calculation required you to merge several DDBs with mrgddb.

You get a truncated version of the frequencies and the dynamical matrix at the end of each (truncated) run, but this warns you that it is not the full dynamical matrix:

chkph3 : WARNING -
The dynamical matrix was incomplete : phonon frequencies may be wrong ...

In Anaddb, I cannot find how to specify only a single q-point rather than a grid (unless of course it's at Gamma and ngkpt can be set to 1 1 1)

So, how do I get the frequencies in this case?