MOLECULAR_MOMENTS localization with DFT+U and UKS - CP2K 2026.1

[Matthew Emerson]

Dear CP2K developers and users,

I'm running into an issue with MOLECULAR_MOMENTS after Wannier localization and was wondering if anyone has seen something similar.

I'm using UKS (also reproduced with DFT+U enabled) together with:

&LOCALIZE
 METHOD JACOBI
 USE_HISTORY F
 &PRINT &MOLECULAR_MOMENTS LOW
    ORDER 1
 &END MOLECULAR_MOMENTS
   &WANNIER_CENTERS
     IONS+CENTERS
   &END WANNIER_CENTERS
 &END PRINT &END LOCALIZE

The SCF converges normally, the Jacobi localization converges, and the printed Wannier centers for both spin channels are finite and appear reasonable. However, the MOLECULAR_MOMENTS output consists entirely of NaN values for every atom/molecule.

As a sanity check, I wrote a small post-processing script that assigns the printed Wannier centers to the nearest atom (using minimum-image PBC) and reconstructs per-atom electronic dipoles which are all reasonable.

Has anyone encountered this before, or is there a known issue with MOLECULAR_MOMENTS for localized UKS wavefunctions? 

[Thomas Kühne]

Dear Matthew,

I found a bug in the MOLECULAR_MOMENTS code path that explains this behavior. It is not caused by UKS, DFT+U, the SCF, or the Jacobi localization itself. 

The finite Wannier centers are consistent with this diagnosis.

After forming the moment matrix product, CP2K summed the entire local allocation of a distributed matrix. This allocation can contain unused padding rows or 

columns that are not initialized or written by the matrix operation. Depending on the MPI distribution and memory contents, a NaN in this padding can therefore 

contaminate all molecular moments.

I have opened a fix:

https://github.com/cp2k/cp2k/pull/5680

The fix restricts the sum to the actual local matrix dimensions. It also extends the existing UKS molecular-moments regression test and makes the numerical test 

matcher reject NaN and Inf explicitly.

I tested the UKS case with 1, 2, and 4 MPI ranks. All runs produced identical finite molecular moments.

Your post-processing of the finite Wannier centers is a reasonable temporary workaround for first moments. Running with a different MPI layout or a single MPI 

rank may also happen to avoid the NaNs, but that is not a reliable fix.

Best regards,

Thomas

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