[CP2K 8.1 CDFT] Exact nonperturbative Becke/Hirshfeld weight export without entering outer SCF

[Dayu Zhu]

# CP2K 8.1 support request: exact nonperturbative Becke/Hirshfeld CDFT weight-grid export

## Build identity

- CP2K version: `8.1`

- Executable revision: `git:0b61f2f`

- Container release-source HEAD: `0bfbd41a7402152da7b86a103d5d95a4184367d9`

- Release tag: `v8.1.0`

- Wrapper SHA256: `c786e567853dbe8fafb35a91ec25ba1ca8f5ea5c02d30f5cdf056719ce9bc2d1`

- Input XML SHA256: `728c139a1d87554490e67f0ab30f1f5c78f944dad5e87671894e1aa833f281f4`

- Container executable: `/cp2k/exe/local/cp2k.psmp`

- Container source root: `/cp2k/src`

The wrapper/account path is intentionally omitted. The hashes above and the

attached sanitized identity CSV are sufficient to identify the inspected

build without exposing an HPC account path.

## Narrow technical need

We need exact CP2K 8.1 Becke and Hirshfeld weight grids for offline integration

of local magnetization from already-converged, unconstrained spin-density

cubes. We want to preserve the original density and avoid running CDFT or an

electronic SCF merely to obtain the geometric/partition weight grid.

This request concerns method semantics and a minimal diagnostic interface

only. It does not claim that stock CP2K is wrong, and it does not request

scientific interpretation of any material.

## Distinctions used in this request

- **Exact weight construction:** running the original CP2K 8.1 Becke or

  Hirshfeld builder with fixed geometry, cell/PBC, grid, atom group, COEFF,

  atom-kind, and partition parameters.

- **Exact weight printing:** writing that already-constructed CP2K PW grid

  through the original `cdft_print_weight_function`/cube-writer path.

- **Nonperturbative measurement:** obtaining the weight without changing or

  reoptimizing the electronic density.

- **CDFT outer-SCF perturbation:** adding `weight * STRENGTH` to the

  Kohn-Sham potential and updating the electronic state/multiplier.

## Exact source references inspected

| Routine | CP2K 8.1 source lines |

|---|---|

| `read_constraint_definitions` | `qs_cdft_utils.F:534-717` |

| `becke_constraint_init` | `qs_cdft_utils.F:100-449` |

| `becke_constraint_low` | `qs_cdft_methods.F:142-581` |

| `hirshfeld_constraint_init` | `qs_cdft_utils.F:966-1086` |

| `create_shape_function` | `hirshfeld_methods.F:141-224` |

| `hirshfeld_constraint_low` | `qs_cdft_methods.F:1153-1365` |

| `qs_ks_cdft_constraint` | `qs_ks_apply_restraints.F:62-128` |

| `cdft_constraint_integrate` | `qs_cdft_methods.F:772-954` |

| `cdft_constraint_print` | `qs_cdft_utils.F:1095-1249` |

| `cdft_print_weight_function` | `qs_cdft_utils.F:1257-1301` |

| `qs_ks_build_kohn_sham_matrix` | `qs_ks_methods.F:207-899` |

| `sum_up_and_integrate` | `qs_ks_utils.F:1195-1625` |

| `cdft_scf` | `qs_scf.F:934-1144` |

| `outer_loop_gradient` | `qs_outer_scf.F:102-174` |

| `outer_loop_update_qs_env` | `qs_outer_scf.F:386-430` |

| `cdft_control_release` | `qs_cdft_types.F:419-493` |

`cdft_print_hirshfeld_density` is not present in the exact `/cp2k/src` tree;

both weight types use the common `cdft_print_weight_function` route.

Our revision-pinned source audit indicates that the low-level Becke/Hirshfeld

builders construct weights before `cdft_constraint_integrate` reads the

current density. The stock weight print is reached only after `cdft_scf`

outer-loop initialization, although the print call precedes the first

constraint-potential addition in that Kohn-Sham build. We request confirmation

of the supported semantics and interface rather than asserting undocumented

API guarantees.

## Questions

1. For CP2K 8.1, is there a supported stock-input route to initialize and

   print exact Becke/Hirshfeld CDFT weight grids without entering the CDFT

   outer-SCF loop?

2. For fixed geometry, real-space grid, atom group, COEFF, atom-kind, and

   partition parameters, are the Becke/Hirshfeld weight grids completely

   independent of the current electron density, current spin density, TARGET,

   STRENGTH, and current Lagrange multiplier?

3. Does `PROGRAM_RUN_INFO/WEIGHT_FUNCTION` print exactly the same weight grid

   regardless of the constrained electronic solution, even though the print

   call occurs after outer-loop initialization?

4. Is there an officially recommended internal routine or minimal driver for

   invoking the original CP2K 8.1 Becke/Hirshfeld weight builders and cube

   writer without running CDFT or electronic SCF?

5. Would a minimal source hook that constructs the weight, writes it, and

   exits before `cdft_constraint_integrate`, constraint-potential building,

   electronic SCF, or CDFT outer-SCF optimization be considered semantically

   exact?

6. For Hirshfeld CDFT in CP2K 8.1, what implementation limitations affect

   ENERGY-only local-magnetization integration, weight printing, forces,

   geometry optimization, and MD?

7. Can multiple `ATOM_GROUP` weight grids, for example R0/R1/R2 and a separate

   HSO4 monitoring region, be generated in one invocation without activating

   coupled constraints?

8. What metadata must be preserved to guarantee exact offline integration of

   `integral w(r)[rho_alpha(r)-rho_beta(r)] dr` against existing spin-density

   cubes, including grid origin, axes, stride, cell/PBC, atom order/kinds,

   partition settings, group/COEFF mapping, and numeric print precision?

## Candidate minimal interface for review

After normal input, subsystem, `qs_env`, MGRID, and PW-pool initialization:

1. allocate one weight PW grid per group using the stock lifetime pattern;

2. call the original Becke or Hirshfeld `init` and low-level builder;

3. call the original common weight cube writer;

4. hash/write provenance metadata;

5. release all temporary grids; and

6. exit before density integration, Kohn-Sham potential construction,

   electronic SCF, or outer-SCF optimization.

No patch, build, input, or calculation is included with this request.

## Lightweight attachments

- `CP2K81_CDFT_weight_evaluator_support_request.md`

- `CP2K81_CDFT_weight_evaluator_minimal_context.txt`

- `CP2K81_exact_source_identity_for_support.csv`

- `CP2K81_CDFT_weight_call_graph_compact.csv`

- `CP2K81_weight_function_dependency_matrix.csv`

- `CP2K81_weight_print_timing_audit.csv`

- `CP2K81_exact_weight_extraction_options.csv`

- `CP2K81_exact_weight_source_excerpts_compact.txt`

No coordinates, WFN, trajectory, cube, binary, full source tree, proprietary

data, or unrelated project details are attached.

[Dayu Zhu]

Dear CP2K developers and users,

I am following up with a much shorter version of my question.

For a fixed geometry, grid, and ATOM_GROUP definition, I need to export the exact CP2K Becke or Hirshfeld CDFT weight function without perturbing an already-converged electronic density.

Could you please clarify:

1. Is there a stock CP2K input option, in version 8.1 or a newer release, that prints the weight-function grid without running the CDFT outer SCF?

2. If not, would it be methodologically valid to add a minimal diagnostic hook that calls the existing CP2K weight builder and cube writer, and then exits before density integration, Kohn–Sham construction, constraint-potential application, and CDFT outer-SCF optimization?

A brief yes/no answer or a pointer to the relevant routine, example, or newer implementation would be very helpful.

The longer message below contains the source-level details, but no detailed review of those attachments is necessary to answer these two questions.

Thank you very much for your time.

Best regards,
Dayu Zhu

[Thomas Kühne]

Dear Dayu,

Am 30.07.2026 um 04:37 schrieb 'Dayu Zhu' via cp2k <cp...@googlegroups.com>:

Dear CP2K developers and users,

I am following up with a much shorter version of my question.

For a fixed geometry, grid, and ATOM_GROUP definition, I need to export the exact CP2K Becke or Hirshfeld CDFT weight function without perturbing an already-converged electronic density.

Could you please clarify:

1. Is there a stock CP2K input option, in version 8.1 or a newer release, that prints the weight-function grid without running the CDFT outer SCF?

1. No. CP2K 8.1, as well as the current version, provides CDFT%PROGRAM_RUN_INFO%WEIGHT_FUNCTION, but this print key is only reached from an active CDFT calculation. There is currently no stock builder-only or print-only input mode that avoids the electronic/CDFT SCF path

2. If not, would it be methodologically valid to add a minimal diagnostic hook that calls the existing CP2K weight builder and cube writer, and then exits before density integration, Kohn–Sham construction, constraint-potential application, and CDFT outer-SCF optimization?

1. Yes. For fixed geometry, cell, grid, atomic kinds and ATOM_GROUP definitions, the Becke and Hirshfeld weight functions are independent of the current molecular electron density. For SHAPE_FUNCTION DENSITY, the Hirshfeld builder uses isolated-atom reference densities rather than the converged molecular density

A brief yes/no answer or a pointer to the relevant routine, example, or newer implementation would be very helpful.

The longer message below contains the source-level details, but no detailed review of those attachments is necessary to answer these two questions.

A clean diagnostic implementation could initialize the normal QS/PW grid environment, allocate the group weight grids as done in qs_ks_cdft_constraint, call the existing constraint-specific initialization and low-level builder, write the grids with cdft_print_weight_function, and then return through normal cleanup before scf_env_do_scf. In CP2K 8.1 the low-level builders and cube writer are private, so a small public wrapper or refactoring would be required.

For an unsubsampled grid, STRIDE 1 1 1 should be used. Please also note that the standard cube format is text-formatted and therefore does not preserve the full internal double precision.

Best regards, 

Thomas Kühne

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