[CP2K-user] [CP2K:22251] Calcite crystal RPBE force convergence

['Kit Joll' via cp2k]

Dear Thomas,

Thank you again for your help. I was able to reproduce your zero-field 
results to approximately the same accuracy with a master version cloned on 
Friday 22nd May. I presume the remaining small differences are due to 
build/library differences.

However, while the zero-field force sum is now well converged, I am not 
seeing the same behaviour once a finite periodic electric field is applied.

For example, using

METHOD GAPW_XC
GAPW_ACCURATE_XCINT
USE_FINER_GRID T
FINE_GRID_FACTOR 2.0

together with a periodic Berry-phase electric field of intensity 2.5e-4 
a.u. along z, the force sum becomes

FORCES| Sum -6.11045615E-09 7.87254948E-09 2.04962359E-04
FORCES| Total atomic force 2.04962360E-04

So the transverse components remain well converged, but the component along 
the applied field direction returns to the order of 1e-4 Ha/Bohr. I see 
similar behaviour when applying the field along other directions. The issue 
also persists when increasing FINE_GRID_FACTOR from 2 to 8: the force-sum 
error remains orders of magnitude larger along the field axis (however, all 
components sum do decrease by ~1 order of magnitude relative to 
FINE_GRID_FACTOR 2). 

Do you know whether there are additional grid, Berry-phase, or 
Wannier-related settings that need to be converged for finite-field force 
calculations? Or is there another setting that should be used together with 
GAPW_XC and GAPW_ACCURATE_XCINT in the presence of a periodic electric 
field?

Please find attached a minimal input (start.inp) /output (output.out) pair 
that reproduces the issue.

Thank you again for your help and advice.

All the best,
Kit
On Thursday, May 21, 2026 at 10:48:00 AM UTC+1 Kit Joll wrote:

> 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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