[Zanthoxylum]

Summary

The core-charge math for the non-linear core correction was implemented
three times: the ground-state module_charge::set_rho_core, the DFPT
first-order response inside DFPT_Rho::compute_drhoc_g, and the DFPT
dynmatcc real-space route accum_nlcc_diag. This PR consolidates the
core-charge math in module_charge so each piece exists once:

• module_charge::set_drho_core (new, chg_tools): the q-shifted
  first-order core response moves verbatim out of
  DFPT_Rho::compute_drhoc_g — the single-atom structure-factor phase
  exp(-i(G+q)tau) and the -i tpiba (G+q)_dir derivative around the
  shared radial kernel. DFPT_Rho::compute_drhoc_g becomes a thin
  delegate; the public API and every call site are unchanged. Design
  note (documented on the interface): unlike set_rho_core, it
  evaluates per G — |G+q| differs at every ig, so the gg_uniq
  shell cache of the q = 0 path cannot be reused.
• module_charge::core_charge_form_factor (new, chg_tools): the
  numeric Simpson radial transform
  4pi/Omega Int r^2 rho_atc(r) j0(g r) dr, previously duplicated
  inline in both the first-order response and accum_nlcc_diag; both
  now call it, and the per-point Bessel/Simpson includes drop out of
  the DFPT translation units.
• The ground-state set_rho_core keeps its cached, multithreaded
  non_linear_core_correction (numeric + analytic shells) — that
  structure does not fit the per-point q-shifted paths; the boundary is
  documented on the new interface.
• The DFPT test targets that compile dfpt_rho.cpp directly
  (rho/phon/xc_analytic serial, pw_run) now also compile
  chg_tools.cpp — the delegate's cross-module call is otherwise an
  undefined reference in those directly-linked test binaries.

The layering follows existing precedent (source_pw/module_pwdft
already includes source_estate; dfpt_rho.cpp already included
chg_tools.h).

What did NOT change

• No behavior change: the moved bodies are verbatim; the
  DrhocLocalizesAtTheAtomNotItsMirror phase regression (localization
  • exp(-i q.tau) phase check at generic q) passes without touching
    the test.

Verification

cmake --build build-gcc13 --target abacus_basic_para MODULE_DFPT_rho_serial \
  MODULE_DFPT_phon_serial MODULE_DFPT_xc_analytic_serial MODULE_DFPT_pw_run_test -j 20
ctest --test-dir build-gcc13 -R "MODULE_DFPT"      # 10/10 targets Passed

• The affected test binaries were deleted and rebuilt from scratch
  (make exit code 0 on every target); full MODULE_DFPT suite
  10/10.
• Anchor against the pre-change baseline (branch binary): bn_hex_G
  full DFPT rerun (the nlcc-bearing wurtzite case; exercises both the
  first-order response and the dynmatcc route) — phonon frequencies and
  the QE-format dyn.q0 output bit-identical to the pre-change
  results.

Documentation

No docs change needed: internal reuse refactor, no INPUT, interface, or
behavior change.

[ABACUS-PR-BOT]

The extracted NLCC formulas match the previous implementation by static comparison. No builds or runtime tests were run; the reported test and bit-identical results were not independently verified.

Non-blocking: The unchanged DrhocLocalizesAtTheAtomNotItsMirror phase check in dfpt_rho_serial_test.cpp:865-879 predates this PR, but is cited as validation. Its extra /lat0 changes the expected phase from about -0.02851 to -0.01509 rad; omitted and reversed q-only phases are then about 0.015 and 0.044 rad from that expectation, both within the 0.05 tolerance. Please correct the expected phase and make the assertion distinguish those cases. The moved production phase is correct.

[ABACUS-PR-BOT]

The scratch-reuse and unit-documentation follow-ups are addressed, and the CMake merge retains the required source entries. The earlier phase-test suggestion remains open; one additional magnitude-test suggestion is below. Static review of 5b7133953: no builds or runtime tests were run, and the author-reported results were not independently verified.

[ABACUS-PR-BOT]

Static incremental review at 89e7a27 covered the Gaussian reference, Simpson mesh, scratch reuse and test registration. I did not run builds or tests.