[CP2K-user] [CP2K:22246] Calcite crystal RPBE force convergence

['Kit Joll' via cp2k]

Dear Thomas,

Many thanks for the very quick and detailed reply! This is extremely 
helpful, and your diagnosis makes a lot of sense given the behaviour we 
were seeing.

I will try compiling the current CP2K master using the new make_cp2k.sh 
workflow, run the relevant regression tests, and then first reproduce your 
zero-field force-sum results on the same calcite frame. After that, I will 
check whether the same settings also resolve the sensitivity we observe in 
the finite-field response tensors.

Thank you also for pointing out the PRINT_LEVEL LOW workaround and the 
relevant GAPW_XC / GAPW_ACCURATE_XCINT settings. I will report back once I 
have tested these on our side.

Best wishes,
Kit

On Wednesday, May 20, 2026 at 9:00:47 PM UTC+1 Thomas Kühne wrote:

> Dear Kit,
>
> Thank you for the detailed report and for the input/output files. I was 
> able to
> reproduce the force-sum residual for the provided calcite frame:
> GPW baseline: Sum F = ( 3.48675295E-05, 1.12518883E-04, -5.39277596E-05 ) 
> Ha/Bohr |Sum F| = 1.29554803E-04 Ha/Bohr
>
> The behavior originate from the XC/grid treatment:
> GPW + USE_FINER_GRID T, FINE_GRID_FACTOR 2.0: |Sum F| = 2.26659720E-05 
> Ha/Bohr GPW + finer/smoothed XC grid: |Sum F| = 1.10078684E-06 Ha/Bohr 
> GAPW_XC: |Sum F| = 4.20880179E-05 Ha/Bohr GAPW_XC + GAPW_ACCURATE_XCINT: |Sum 
> F| = 1.68750841E-07 Ha/Bohr GAPW + GAPW_ACCURATE_XCINT + USE_FINER_GRID 
> T, FINE_GRID_FACTOR 2.0: |Sum F| = 3.85198029E-08 Ha/Bohr GAPW_XC + 
> GAPW_ACCURATE_XCINT + USE_FINER_GRID T, FINE_GRID_FACTOR 2.0: |Sum F| = 
> 1.04105369E-08 Ha/Bohr
>
> This strongly suggests that the residual is dominated by an 
> egg-box/grid-coupling
> effect in the XC/grid integration rather than by pseudopotentials, basis 
> sets, or
> SCF noise.
>
> For this system, the best tested setting was:
> &QS EPS_DEFAULT 1.0E-18 METHOD GAPW_XC GAPW_ACCURATE_XCINT &END QS &XC DENSITY_CUTOFF 
> 1.0E-10 GRADIENT_CUTOFF 1.0E-10 TAU_CUTOFF 1.0E-10 &XC_GRID FINE_GRID_FACTOR 
> 2.0 USE_FINER_GRID T &END XC_GRID &XC_FUNCTIONAL NO_SHORTCUT &GGA_C_PBE &END 
> GGA_C_PBE &GGA_X_RPBE &END GGA_X_RPBE &END XC_FUNCTIONAL &END XC
>
> METHOD GAPW with the same GAPW_ACCURATE_XCINT and fine XC grid also 
> performs
> very well, but in this test GAPW_XC was slightly better and should be the 
> less
> intrusive change relative to the original GPW calculation.
>
> One practical note: GAPW_ACCURATE_XCINT has been improved recently, so I 
> would
> recommend testing with a recent CP2K master or a release that includes 
> those
> changes. Also, for this calcite input current master hit a post-SCF output 
> crash
> with PRINT_LEVEL MEDIUM; using PRINT_LEVEL LOW while keeping explicit
> &FORCES ON avoided that and still printed the force block.
>
> Best wishes,
> Thomas
>

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