# Dynamical matrix and phonon frequency

**URL:** <https://discourse.abinit.org/t/dynamical-matrix-and-phonon-frequency/3768>\
**Category:** Ground state\
**Created:** [October 8, 2025, 4:01pm UTC](https://discourse.abinit.org/t/dynamical-matrix-and-phonon-frequency/3768 "2025-10-08T16:01:59Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Dom\_7](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/d/7bcc69/32.png) [@Dom\_7](https://discourse.abinit.org/u/Dom_7)\
**Post date:** [October 8, 2025, 4:01pm UTC](https://discourse.abinit.org/t/dynamical-matrix-and-phonon-frequency/3768/1 "2025-10-08T16:01:59Z")

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Hi all,

I was doing few phonon calculations at finite-q points. In the output file we get the dynamical matrix

```auto
 Dynamical matrix, in cartesian coordinates,
   if specified in the inputs, asr has been imposed
     j1 j2 matrix element
  dir pert dir pert real part imaginary part

```

To get the phonon energy in Hatree, do we just take the square root of the eigen values of the dynamical matrix obtained from the above segement?

The values I obtained were different when I manually tried to diagonalise this matrix compared to the phonon energies obtained in the ABINIT run. Is my understanding correct? I am attaching the ABINIT output file and the python script I was using to get the eigen values here.  
[phonon.abo](https://discourse.abinit.org/uploads/short-url/zmZcIon8Q8kzTbwOdsNE38LmwK3.abo) (66.0 KB)  
[dynamical.py.txt](https://discourse.abinit.org/uploads/short-url/wdGcFQY3oKUlwbH2HtjMm9ABWOW.txt) (3.6 KB)

Thanks  
Dominic

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<div class="post-metadata">

**Author:** ![Dom\_7](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/d/7bcc69/32.png) [@Dom\_7](https://discourse.abinit.org/u/Dom_7)\
**Post date:** [October 8, 2025, 5:00pm UTC](https://discourse.abinit.org/t/dynamical-matrix-and-phonon-frequency/3768/2 "2025-10-08T17:00:28Z")

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I realized the issue was with the unit conventions — the ion masses were given in atomic mass units (amu), while the force constants were expressed in Hartree/bohr². In Hartree units, mass is defined relative to the electron mass, so 1 amu~ 1840 me.  
When converting consistently, taking the square root of this mass ratio brings the phonon frequencies into perfect agreement with the values obtained from ABINIT.

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**Author:** ![bguster](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/b/ac8455/32.png) [@bguster](https://discourse.abinit.org/u/bguster)\
**Post date:** [October 13, 2025, 8:58am UTC](https://discourse.abinit.org/t/dynamical-matrix-and-phonon-frequency/3768/3 "2025-10-13T08:58:22Z")

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Hi Dominic,

Just a comment to your post: ABINIT uses atomic units across the board unless specified otherwise.

Good luck!

Bogdan