Goal

Turn a CPMD input deck into a CPMDParams message file that cpmdc_session_create(), rgpot, or eOn’s params_path can load. The message carries method setup only. Coordinates, species, and the cell of each step travel in ForceInput.

Write the message as Cap’n Proto text

Cap’n Proto text is the shortest way to write a message by hand, and the capnp tool encodes it without any code. This message describes a closed-shell Si and N cluster in a 14 Angstrom cube, BLYP at 70 Ry, with Kleinman-Bylander pseudopotentials:

(
  functional = "BLYP",
  cutOffRy = 70.0,
  inputSections = [
    ( cpmd = ( optimizeWavefunction = true,
               convergenceOrbitals = 1.0e-5,
               maxIter = 400,
               odiisVectors = 10,
               centerMoleculeOff = true ) ),
    ( system = ( symmetry = 1, angstrom = true, cutOffRy = 70.0,
                 cell = [14.0, 1.0, 1.0, 0.0, 0.0, 0.0] ) ),
    ( dft = ( functional = "BLYP", gcCutoff = 1.0e-7, newCode = true ) ),
    ( atoms = ( pseudopotentials = [
        ( element = "Si", path = "Si_MT_BLYP.psp", lmax = 2, loc = 2,
          kleinmanBylander = true ),
        ( element = "N", path = "N_MT_BLYP.psp", lmax = 1, loc = 1,
          kleinmanBylander = true )
      ] ) )
  ]
)

Save it as cluster.params.txt and encode it against the schema in the cpmdc checkout:

capnp encode schema/Potentials.capnp CPMDParams \
  < cluster.params.txt > cluster.params.bin

capnp encode writes the unpacked flat stream that every cpmdc entry point reads. It rejects a field name that is not in the schema, so a typo fails here and not inside CPMD.

Map deck lines to fields

Each deck line has one preferred carrier. Look for a typed field first; the option mapping lists every one of them with its feature ID.

Deck line	Section	Field
OPTIMIZE WAVEFUNCTION	cpmd	optimizeWavefunction = true
CONVERGENCE ORBITALS / 1.0d-5	cpmd	convergenceOrbitals = 1.0e-5
MAXITER / 400	cpmd	maxIter = 400
ODIIS / 10	cpmd	odiisVectors = 10
PCG MINIMIZE	cpmd	directives = [ ( keyword = "PCG MINIMIZE", args = [] ) ]
CENTER MOLECULE OFF	cpmd	centerMoleculeOff = true
SYMMETRY / 1	system	symmetry = 1
CELL / 14.0 1.0 1.0 0.0 0.0 0.0	system	cell = [14.0, 1.0, 1.0, 0.0, 0.0, 0.0]
POISSON SOLVER HOCKNEY	system	poissonSolver = "HOCKNEY"
CUTOFF / 70	system	cutOffRy = 70.0
FUNCTIONAL BLYP	dft	functional = "BLYP"
NEWCODE	dft	newCode = true
GC-CUTOFF / 1.0d-7	dft	gcCutoff = 1.0e-7
*Si_MT_BLYP.psp KLEINMAN-BYLANDER / LMAX=D LOC=D	atoms	( element = "Si", path = "Si_MT_BLYP.psp", lmax = 2, loc = 2, kleinmanBylander = true )

lmax, loc, and skip take the channel as an integer: 0 renders S, 1 P, 2 D, 3 F. A keyword without a typed field goes into that section’s directives list: keyword is the line, and each entry of args becomes one indented line below it.

Defaults that change the physics

Four defaults decide what CPMD computes when the message leaves them out.

Left out	What cpmdc renders	Consequence
the system section	SYMMETRY 0, ANGSTROM, CUTOFF from cutOffRy, no CELL	an isolated cluster; the cell comes from ForceInput.box
symmetry and poissonSolver	SYMMETRY 0 plus POISSON SOLVER HOCKNEY	the Hockney isolated-system solver, even for a periodic box
maxIter	no MAXITER line; the OpenCPMD path then inserts MAXITER 40 into the first deck	an SCF that needs more than 40 steps fails the call
kleinmanBylander	the pseudopotential line without KLEINMAN-BYLANDER	Gauss-Hermite integration of the nonlocal projectors, a different energy

Set each of them on purpose. ForceInput.box supplies CELL only when the deck has no CELL line.

Check the rendered deck

CPMDC_DECK_OUT writes the deck cpmdc renders from the message, so the check needs no OpenCPMD:

CPMDC_DECK_OUT=cluster.inp build/example_host_step \
  cluster.params.bin step.bin
cat cluster.inp

step.bin is any encoded ForceInput; the default build evaluates it with the reference evaluator after the deck is written. Compare cluster.inp line by line against the deck you started from. On the OpenCPMD path a second deck, the one CPMD parses with the geometry merged in, overwrites the first; the deck debugging how-to shows both.

Where the geometry-dependent lines go

The OpenCPMD path writes &ATOMS itself from each step’s atomic numbers and positions. An element listed in atoms.pseudopotentials takes its pseudopotential file, LMAX, LOC, and KLEINMAN-BYLANDER from that entry. KLEINMAN-BYLANDER is written on the *file line, which is where CPMD reads it, and only when that entry sets kleinmanBylander = true. The rendered method deck marks each listed entry with a !SPECIES SYM comment so the geometry merge can match the file to an atomic number. An element the message does not list uses this table:

Element	File	Channels
H	H_CVB_BLYP.psp	LMAX=S
C	C_MT_BLYP.psp	LMAX=P
N	N_MT_BLYP.psp	LMAX=P
O	O_MT_BLYP.psp	LMAX=P
Si	Si_MT_BLYP.psp	LMAX=D LOC=D
Ge	Ge_MT_BLYP.psp	LMAX=P

An element in neither the message nor the table fails the evaluation. The message names the atomic number. A method deck whose &ATOMS block already has coordinate lines, for example from inputBlocks, is passed to CPMD unchanged; each step then overwrites only the positions.