# Calculation of the interatomic forces fails

**URL:** <https://discourse.abinit.org/t/calculation-of-the-interatomic-forces-fails/3660>\
**Category:** Response calculations\
**Created:** [February 12, 2025, 5:20am UTC](https://discourse.abinit.org/t/calculation-of-the-interatomic-forces-fails/3660 "2025-02-12T05:20:11Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![rajeshatpsi](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rajeshatpsi](https://discourse.abinit.org/u/rajeshatpsi)\
**Post date:** [February 12, 2025, 5:20am UTC](https://discourse.abinit.org/t/calculation-of-the-interatomic-forces-fails/3660/1 "2025-02-12T05:20:11Z")

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Hi,

Could someone please check this electro phonon interatomic forces calculation which throws an error saying:

_message: |_

- Information is missing in the DDB.\*
- The dynamical matrix number 6 cannot be built,\*
- since no block with qpt: 5.000000E-01 0.000000E+00 5.000000E-01\*
- has been found.\*
- Action: add the required block in the DDB, or modify the q-mesh your input file.\*

However, dataset 2,3 and 4 calculates for the q points (000) (0.5 0 0) and (0 0 0.5)! Why is it looking for a block with qpt (0.5 0 0.5)?  
It is hexagonal lattice P6/mmm. I attach all the input files and log files here.

Best,  
Rajesh  
[B2Mo\_4.abi](https://discourse.abinit.org/uploads/short-url/N42o4cVUu8NMybNCEp1XhX7jKn.abi) (1.1 KB)  
[B2Mo\_3.abi](https://discourse.abinit.org/uploads/short-url/iyl8hVpwkHJpXzgHObkLaoIvDmL.abi) (414 Bytes)  
[B2Mo\_2.abi](https://discourse.abinit.org/uploads/short-url/2oVb6EAmJz9CyCLAh8CoBA3W9cq.abi) (87 Bytes)  
[log4.abi](https://discourse.abinit.org/uploads/short-url/6Z3x8v3rdyAreaFItqHMT0ZcFH.abi) (4.1 KB)  
[B2Mo.abi](https://discourse.abinit.org/uploads/short-url/3qCT1OArtW2R1u0LHkx0oALBVSO.abi) (4.0 KB)

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**Author:** ![mverstra](https://discourse.abinit.org/user_avatar/discourse.abinit.org/mverstra/32/92_2.png) [@mverstra](https://discourse.abinit.org/u/mverstra)\
**Post date:** [February 12, 2025, 9:46am UTC](https://discourse.abinit.org/t/calculation-of-the-interatomic-forces-fails/3660/2 "2025-02-12T09:46:23Z")

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You changed the structure and symmetries, so you can not expect the list of irreducible q and perturbations to remain the same: your input copies those for Al FCC, but the number and coordinates of the q will change.

Please check the tutorial and see how to generate the list of irred q. The simplest is to run a dummy GS calculation with kptopt 1 and ngkpt = your qpt grid. This will give you the list of irred q in the kpt variable (no need to converge or anything).

Check the new eph tutorial as well: It explains you can use qptopt 1 and iqpt with the index of the q you want.

Finally, have a look at the abipy tutorials. There are workflows which do all of this indexing for you and avoid forgetting perturbations etc.

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**Author:** ![rajeshatpsi](https://discourse.abinit.org/letter_avatar_proxy/v4/letter/r/34f0e0/32.png) [@rajeshatpsi](https://discourse.abinit.org/u/rajeshatpsi)\
**Post date:** [February 13, 2025, 7:00pm UTC](https://discourse.abinit.org/t/calculation-of-the-interatomic-forces-fails/3660/3 "2025-02-13T19:00:14Z")

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Hi mverstra,

Thanks for the reply. I have checked with your 1st suggestion, but there is a confusion about generated list of irred q. When I use different ngqpt from ngkpt I get same list. Seemingly, the list is depending only on ngkpt value.

For example, with ngqpt 222 and ngkpt 666 input it gives the same list if I use ngqpt 666 and ngkpt 666.

Here I attach my input and output files.

[B2Mo\_ngqpt222.abo](https://discourse.abinit.org/uploads/short-url/2PyJ2gA45d24Nfq8uJmMA0X7ua7.abo) (74.1 KB)  
[B2Mo.abi](https://discourse.abinit.org/uploads/short-url/pHZ0eSfc5f41uAWtHWix7bFj4a7.abi) (2.2 KB)  
[B2Mo\_ngqpt666.abo](https://discourse.abinit.org/uploads/short-url/h1fNXoJYnItmIqkQyDvMQOnkjIo.abo) (74.1 KB)

regards,  
Rajesh