Add fixed-cell i-PI socket driver with validated property responses

[Phorbol]

What's changed?

Add an opt-in, persistent fixed-cell i-PI socket client for repeated external energy, force, and stress evaluations. ABACUS uses calculation scf with socket_driver true; Relax_Driver dispatches to Socket_Driver. Atomic positions can change, but cell, species, atom count, and electronic-structure settings must remain fixed.

The client reads results directly from the completed ESolver frame, not from output files. POSDATA updates positions and runs SCF, then publishes energy, enabled derivatives, and convergence metadata together. GETFORCE consumes and clears that frame. Input validation covers finite values, atom counts, cell conditioning and inverse consistency, including ASE's inverse-cell layout. Wire integers use int32.

Energy is always computed. cal_force and cal_stress independently enable derivatives; disabled fields are protocol padding, with presence declared in abacus.socket.properties.v1 extras. SCF nonconvergence is reported as scf_converged=false; available results are returned and the caller chooses an acceptance policy. Stress is supported at a fixed cell; variable-cell updates, NPT, and cell optimization are excluded.

The ASE AbacusSocketIO adapter validates metadata and results, preserves atom ordering, enables requested properties, and restarts the client if a later request expands the property set. New requests clear cached results and convergence metadata; only fully validated responses are published.

Usage and compatibility

INPUT_PARAMETERS
calculation scf
socket_driver true
cal_force true
cal_stress false

Set ABACUS_SOCKET_ADDRESS=localhost:31415 for TCP or ABACUS_SOCKET_ADDRESS=/tmp/ipi_abacus:UNIX for a UNIX socket. AbacusSocketIO selects the endpoint automatically. No additional C++ socket library is required.

Generic i-PI clients must explicitly enable all derivatives they consume: omitted force/stress switches are disabled. Clients ignoring extras cannot distinguish padding from a physical zero or read the convergence flag. AbacusSocketIO requires presence metadata for force/stress; legacy empty extras are accepted only for energy-only results. This is not unconditional backward compatibility with older ABACUS socket binaries.

INPUT Parameter Changes

• Add socket_driver, restricted to calculation scf.
• In socket mode, chg_extrap default selects first-order.
• cal_force is not forced on; force and stress switches remain independent.
• Updated docs/parameters.yaml, docs/advanced/input_files/input-main.md, and ASE documentation.

Core Module Impact

Socket code is in source/source_relax, registered in both CMake and Makefile object lists. KS runner publishes its current convergence state after SCF; LCAO output receives the current convergence argument while retaining the upstream EXX interface. These shared solver changes require CI regression coverage beyond socket tests.

The fixed-cell hardening branch is integrated, including inverse-layout compatibility and frame-consistency fixes. Unrelated ELPA reports and historical design documents are excluded.

Exact Verification Performed

Local CMake build: GNU C++, MPI enabled for the repository test configuration, LCAO/ELPA/libXC disabled, installed FFTW/BLAS/ScaLAPACK and GTest. GTest requires C++17; production socket/parser sources were additionally checked under C++11.

cmake --build /tmp/abacus-pr7609-check/build --target \
  MODULE_RELAX_socket_ipi_test MODULE_RELAX_socket_frame_test \
  MODULE_RELAX_socket_driver_test MODULE_IO_read_item_serial -j 4

OMP_NUM_THREADS=1 ctest --test-dir /tmp/abacus-pr7609-check/build \
  --output-on-failure \
  -R 'MODULE_RELAX_socket_(ipi|frame|driver)_test|MODULE_IO_read_item_serial'

PYTHONPATH=interfaces/ASE_interface \
  /home/gengjianrui/bin/abacus-variable-cell-runtime/venv/bin/python \
  -m unittest abacuslite.core.TestAbacusCalculator -k socket -v

g++ -std=c++11 -fsyntax-only -Isource \
  -Isource/source_base/module_container \
  -I/opt/devtools/fftw/fftw-3.3.10/include \
  source/source_relax/socket_frame.cpp \
  source/source_relax/socket_ipi.cpp \
  source/source_relax/socket_driver.cpp \
  source/source_io/module_parameter/read_inp_sys.cpp

python3 tools/03_code_analysis/agent_governance_check.py \
  --base upstream/develop --head HEAD --format text
git diff --check

Results: 4/4 CTest targets passed; 10/10 ASE socket tests passed; C++11 syntax checks passed. Expanded the actual Makefile OBJS_ABACUS list and verified socket_driver.o, socket_ipi.o, and socket_frame.o appear exactly once. Governance reports header-dependency warnings only.

Regression coverage includes two different consecutive geometries with corresponding energy/force/virial/convergence changes, duplicate GETFORCE rejection, invalid-next-frame rejection, nonorthogonal ASE cell layout, optional properties, and clearing stale ASE results after a failed response. The stale-cache regression failed before the fix and passed afterward.

Limits: driver tests use a fake solver and serial socket execution. No fresh full Intel build, multi-rank socket runtime test, or real-DFT socket-versus-independent-frame comparison was completed for this integrated revision. Full build/integration CI is pending; prior branch numerical results are not evidence for this revision.

Linked Issue

No linked issue; this introduces an optional external-driver interface.

Governance

New headers use standard-library types required by their public declarations and value members. No new default arguments or third-party dependency. Input docs and both build paths are updated. No global-dependency budget blocker was reported; no exception is requested.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.

[mohanchen]

Thanks @Phorbol for your contribution? Could you kindly reply to my questions?

[Phorbol]

@mohanchen I have pushed follow-up changes and replied to the review threads.

The current implementation now:

• uses calculation="scf" with a separate socket_driver boolean input, instead of introducing calculation="socket";
• moves socket execution out of driver_run.cpp into the relax/socket driver path;
• renames the i-PI socket files to socket_ipi.*;
• adds ASE AbacusSocketIO support with an interface example in interfaces/ASE_interface/examples/socketio.py;
• documents ABACUS_SOCKET_ADDRESS endpoint formats, fixed-cell behavior, immutable INPUT/STRU settings, atom ordering, MPI/GPU notes, and socket running_scf.log behavior;
• regenerates input-main.md from parameters.yaml to keep the docs CI check in sync.

A few behavior details are worth noting:

• In socket mode, ABACUS still writes OUT.ABACUS/running_scf.log because calculation remains scf. Since one ABACUS client process is kept alive, multiple socket SCF evaluations from the same AbacusSocketIO instance are appended to the same running_scf.log. The per-step energy/force results are returned through the i-PI socket to ASE; running_scf.log should be treated mainly as the diagnostic log for the socket client. This is documented in docs/advanced/interface/ase.md.
• The i-PI POSDATA message does not carry element symbols. AbacusSocketIO handles this by sorting the internal socket atoms consistently with the STRU species grouping and mapping returned forces back to the original ASE Atoms order. This avoids silent force/atom mismatches for structures read from CIF/extxyz/POSCAR-like formats, and is documented and covered by a unit test.
• The intended use case is high-throughput fixed-cell force/energy evaluation from external drivers, especially ASE workflows(active learning for MLIP or TS search such as ATST) where many structures share the same ABACUS input setup. It can also serve finite-displacement force-evaluation workflows before phonon/thermal-conductivity post-processing, while force-constant fitting/post-processing remains handled by tools such as Phonopy/Phono3py.

Please let me know if any thread still needs further changes.

[Phorbol]

This PR intentionally implements fixed-cell socket evaluations first. The i-PI POSDATA message carries cell data, but changing the ABACUS cell safely would require reinitializing cell-dependent state such as basis/grid/k-points/neighbors and possibly electronic-structure setup. To avoid silent incorrect results, AbacusSocketIO rejects cell changes on the ASE side, and the ABACUS socket driver also checks incoming POSDATA cells against the initial STRU cell and exits if they differ.

The currently supported scope is repeated energy/force evaluation for structures with the same cell, atom count, species, pseudopotentials, basis, k-points, spin setting, and other core INPUT/STRU parameters. Changing any of these requires creating a new AbacusSocketIO instance and launching a new ABACUS client process.

[mohanchen]

Thanks for your contribution, but I still have some questions. It would be easier to chat in another way, could you write an email to me? (mohanchen@pku.edu.cn) @Phorbol

[mohanchen]

LGTM. This is an important contribution, thanks!