[CP2K-user] [CP2K:22253] Calcite crystal RPBE force convergence

'Kit Joll' via cp2k cp2k at googlegroups.com
Wed May 27 12:10:49 UTC 2026
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Dear Thomas,

Thank you again for the helpful suggestions. I have now repeated the 
finite-field calculation with both signs of the field and with the field 
magnitude halved.

All four calculations used the same input settings, cell, coordinates, 
GAPW_XC / GAPW_ACCURATE_XCINT setup, XC grid settings, basis sets and 
pseudopotentials. The only changes were the PERIODIC_EFIELD INTENSITY and 
POLARISATION. The effective field is along z, with E_z = INTENSITY * 
POLARISATION_z. I have attached the four corresponding input/output pairs. 
The attachment filenames indicate the actual field values (the PROJECT_NAME 
inside the input files is just a leftover label).

The resulting force sums are:

E_z / a.u.        Sum F_x          Sum F_y          Sum F_z          (Sum 
F_z) / E_z
-2.5e-4          -6.1085e-09      7.8702e-09     -2.04936242e-04   
0.819744968
-1.25e-4         -6.1090e-09      7.8708e-09     -1.02473941e-04   
0.819791528
+1.25e-4         -6.1100e-09      7.8720e-09      1.02475947e-04   
0.819807576
+2.5e-4          -6.1104e-09      7.8725e-09      2.04962359e-04   
0.819849436

So the absolute z-component of the force-sum residual approximately halves 
when the field is halved, while the ratio Sum F_z / E_z remains essentially 
constant at about 0.8198. The residual also changes sign when the field 
direction is reversed. The transverse components remain at the zero-field 
level, around 1e-8 Ha/Bohr.

The central finite-difference slopes are very similar for the two field 
magnitudes:

[ F_z(+1.25e-4) - F_z(-1.25e-4) ] / [2 * 1.25e-4] = 0.819799552

[ F_z(+2.5e-4) - F_z(-2.5e-4) ] / [2 * 2.5e-4] = 0.819797202

This seems to confirm your suggested diagnostic: the remaining force-sum 
residual is odd in the field, proportional to the applied field, and 
parallel to the field direction. It therefore looks distinct from the 
original XC/grid force issue.

Would you recommend any further diagnostic at this point, or would it be 
useful for me to prepare a more minimal test case for the 
periodic-electric-field force contribution?

All the best,
Kit

On Tuesday, May 26, 2026 at 7:15:40 PM UTC+1 Thomas Kühne wrote:

> Dear Kit,
>
> I agree with your interpretation. The important point is that the residual 
> is now almost entirely parallel to the applied periodic field. In your 
> z-field calculation,
>
> FORCES| Sum -6.11045615E-09 7.87254948E-09 2.04962359E-04
>
> the x and y components remain at the zero-field accuracy, whereas the z 
> component is of the order of the applied field strength. This strongly 
> suggests that 
> we are no longer looking at the same XC/grid egg-box issue that was cured 
> by GAPW_XC + GAPW_ACCURATE_XCINT. Rather, the remaining term seems 
> to originate from the finite-field/Berry-phase force contribution itself.
>
> For a neutral periodic system I would still expect the total force to 
> vanish, also in a homogeneous Berry-phase electric field, up to numerical 
> noise. The 
> residual is also not simply a physical net-charge force. The system is 
> neutral in terms of the GTH valence charges, and the residual is 
> approximately 0.82 
> times the field strength rather than an obvious integer-charge 
> contribution.
>
> I do not think there is a Wannier-localization setting that should fix 
> this. The PERIODIC_EFIELD implementation uses the Berry-phase formulation 
> directly; 
> the printed Wannier/localization controls are not convergence parameters 
> for this force term. Likewise, increasing the XC fine grid should only 
> affect the 
> ordinary XC/grid part. Since the transverse components are already small, 
> and the residual tracks the field direction, I would not expect 
> FINE_GRID_FACTOR 
> alone to remove it completely.
>
> Could you please do two quick checks, if convenient?
>
> 1. Repeat the same calculation with the field reversed, i.e. INTENSITY 
> -2.5e-4 or POLARISATION 0 0 -1.
> 2. Repeat with half the field strength, e.g. 1.25e-4 a.u.
>
> If the force-sum residual changes sign with the field and scales linearly 
> with its magnitude, that would be very strong evidence that the issue is in 
> the periodic 
> electric-field force contribution rather than in the GAPW/XC integration.
>
> At first sight, finite-field force code path looks like a missing or 
> imperfect cancellation between the ionic and electronic Berry-phase force 
> terms under a uniform 
> translation. In other words, it may be a real CP2K bug or at least a 
> limitation of the present PERIODIC_EFIELD force implementation, rather than 
> an 
> input-convergence issue.
>
> As a practical workaround for testing internal forces, subtracting the 
> average force from all atoms would remove the spurious center-of-mass 
> acceleration, but 
> I would not present that as a formally clean solution until we understand 
> the origin of the term.
>
> Best wishes,
> Thomas
>
> Am 26.05.2026 um 16:35 schrieb 'Kit Joll' via cp2k <cp... at googlegroups.com
> >:
>
> 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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