A
=

ABI
   Application binary interface: the functions, types, and calling
   conventions of ``libcpmdc`` as a compiled library.
   ``CPMDC_ABI_VERSION`` and ``cpmdc_abi_version()`` number its
   generations.
BLYP
   Becke exchange with Lee, Yang, and Parr correlation, a generalized
   gradient density functional; the default ``functional`` of
   ``CPMDParams``.

C
=

Calculator
   A group of MPI ranks that runs one CPMD calculation with its own
   communicator and orbitals. Created by ``cpmdc_bind_calculator()``.
Cap'n Proto
   The serialization format of every message ``cpmdc`` reads or writes.
   ``capnp encode`` turns a text message into the binary form.
Cold call
   An evaluation that runs CPMD's full setup and starts the SCF without
   stored orbitals: the first call of a session, a changed cell, or a
   re-configured session.
CPMD
   Car-Parrinello Molecular Dynamics, the plane-wave density functional
   code; ``cpmdc`` embeds its OpenCPMD distribution.
``CPMDParams``
   The message holding method setup: functional, cutoff, and the deck
   sections. One per session.
``CPMDCSession``
   The C handle that owns one ``CPMDParams``, a fixed topology, and the
   results of its last evaluation.

D
=

Deck
   A CPMD input file of ``&SECTION ... &END`` blocks. ``cpmdc`` renders
   one from ``CPMDParams`` and hands it to CPMD in memory.
Default build
   ``libcpmdc.so`` built without OpenCPMD. It evaluates a deterministic
   reference function so the ABI can be tested anywhere.
DFT
   Density functional theory.

E
=

``ener_com``
   The OpenCPMD module that holds the energy terms of the last SCF;
   ``cpmdc_last_energy_components()`` copies it.

F
=

Feature ID
   A stable string naming a capability, such as
   ``abi.cpmdc_session_calculate_result`` or
   ``params.inputSections.cpmd.maxIter``. Looked up with
   ``cpmdc_feature_find()``.
File route
   Running ``cpmd.x`` once per force call and exchanging decks and
   output files through the file system; the alternative to embedding.
``fion``
   OpenCPMD's array of ionic forces, ``coor%fion``, read by ``cpmdc``
   after each SCF.
``ForceInput``
   The message holding one geometry: positions, atomic numbers, an
   optional cell, and output units.

G
=

Gauss-Hermite integration
   CPMD's default numerical treatment of the nonlocal pseudopotential
   projectors, used when ``KLEINMAN-BYLANDER`` is not on the
   pseudopotential file line.

H
=

Hockney solver
   ``POISSON SOLVER HOCKNEY``, CPMD's Poisson solver for an isolated
   system in a box. Added by ``cpmdc`` when ``SYMMETRY 0`` has no
   solver.

K
=

Kleinman-Bylander form
   A separable form of the nonlocal pseudopotential, selected per
   species with ``KLEINMAN-BYLANDER`` on the ``*file`` line, or
   ``kleinmanBylander = true`` in ``CPMDAtomsPseudopotential``.

M
=

``memfd``
   An anonymous in-memory file. ``cpmdc`` writes the composed deck to
   one and passes CPMD its ``/proc/self/fd/N`` path.
MPI
   Message Passing Interface, the library CPMD uses to spread one SCF
   over several processes (ranks).

N
=

NEB
   Nudged elastic band, a method that relaxes a chain of images between
   two minima to find the minimum energy path.

O
=

ODIIS
   Orbital direct inversion in the iterative subspace, CPMD's
   preconditioned DIIS wavefunction optimiser and its default.
OpenCPMD
   The open-source CPMD code base at ``github.com/OpenCPMD/CPMD``.

P
=

Parent rank
   Rank 0 of CPMD's communicator; it writes CPMD's output and holds the
   result the host should use.
PCG
   Preconditioned conjugate gradient wavefunction optimiser.
   ``PCG  MINIMIZE`` adds a line minimisation along each search
   direction.
``PotentialConfig``
   A message with one backend arm, such as ``cpmd``, and an optional
   ``CommonMethodSpec`` overlay; read by ``cpmdc_configure()``.
``PotentialResult``
   The result message: energy, forces, and the extension fields, in the
   units ``ForceInput`` asked for.
Pseudopotential
   A file, such as ``O_MT_BLYP.psp``, replacing the core electrons of an
   element; found through ``CPMDC_PSEUDO_DIR``.

R
=

Reference evaluator
   The deterministic function the default build evaluates instead of
   CPMD.
``RESTART.1``
   CPMD's binary restart file with orbitals, coordinates, and cell; read
   and patched by ``cpmdc-restart``.
rgpot
   The OmniPotentRPC library that loads potential backends, among them
   ``libcpmdc`` through its ``CPMDPot`` frontend.

S
=

SCF
   Self-consistent field: the iterative optimisation of the orbitals for
   fixed nuclei that each force call runs.
Stub
   ``libcpmdc_stub.a``, a link-only build whose calls all fail and whose
   ``cpmdc_available()`` is 0.

T
=

``tau0``
   OpenCPMD's array of ionic positions in Bohr, ``coor%tau0``, written
   by ``cpmdc`` from each step's positions.
Topology
   The atom count and ordered atomic numbers a session accepts on its
   first successful step.

W
=

Warm call
   An evaluation that reuses CPMD's setup and starts the SCF from the
   orbitals of the previous call of the same session.