# Calculation of dynamical matrix from single q-point after mrgddb

**URL:** <https://discourse.abinit.org/t/calculation-of-dynamical-matrix-from-single-q-point-after-mrgddb/83>\
**Category:** Response calculations\
**Created:** [February 19, 2021, 3:59am UTC](https://discourse.abinit.org/t/calculation-of-dynamical-matrix-from-single-q-point-after-mrgddb/83 "2021-02-19T03:59:16Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![jackbaker](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@jackbaker](https://discourse.abinit.org/u/jackbaker)\
**Post date:** [February 19, 2021, 3:59am UTC](https://discourse.abinit.org/t/calculation-of-dynamical-matrix-from-single-q-point-after-mrgddb/83/1 "2021-02-19T03:59:16Z")

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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?

Best,

JSB

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**Author:** ![gmatteo](https://discourse.abinit.org/user_avatar/discourse.abinit.org/gmatteo/32/6_2.png) [@gmatteo](https://discourse.abinit.org/u/gmatteo)\
**Post date:** [July 10, 2021, 11:02pm UTC](https://discourse.abinit.org/t/calculation-of-dynamical-matrix-from-single-q-point-after-mrgddb/83/2 "2021-07-10T23:02:11Z")

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> 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.

In this case, one should use _mrgddb_ to merge all the independent perturbations computed in the different runs and then use _anaddb_ to fill by symmetry all the entries of the dynamical matrix for a given q.

> 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)

Use ifcflag 0 to disable the Fourier interpolation and provide the list of q-points  
where phonons should be computed via nph1l and qph1l.  
In this case, the q-points **must be** in the DDB file:

```auto
ifcflag 0
asr 2
nph1l 1
qph1l
    0.0 0.0 0.0. 1.0 # Frac_coords weight 

```