Goal¶

See the CPMD input deck that cpmdc renders from a CPMDParams message, and work out what the OpenCPMD path changes before CPMD (Car-Parrinello Molecular Dynamics) parses it.

Write the deck to a file¶

Set CPMDC_DECK_OUT to a file name before the process configures cpmdc:

export CPMDC_DECK_OUT=$PWD/method.inp
build/example_host_step params.bin step.bin
cat method.inp

cpmdc writes the file each time it renders a message: in cpmdc_session_create(), cpmdc_session_set_params(), cpmdc_set_params(), the PotentialConfig entry points, and every one-shot call. Each write replaces the previous content, so the file holds the deck of the most recent configuration. The write happens in the default build too, so a message can be checked on a machine without OpenCPMD. An empty CPMDC_DECK_OUT, or a path that cannot be opened for writing, skips the write without an error.

Under mpirun every rank renders the same deck to the same path. Point each rank at its own file when the ranks share a file system, for example through a wrapper script that appends $OMPI_COMM_WORLD_RANK.

What the OpenCPMD path changes¶

The file holds the method deck. On the first evaluation of a session, the OpenCPMD path builds the deck that CPMD parses from it in three steps:

If the method deck has an &ATOMS block with coordinate lines, it is used as it is. Otherwise every &ATOMS block is dropped and a new one is written from the step’s atomic numbers and positions. A !SPECIES SYM comment in the method deck selects that element’s file, LMAX, LOC, and, when the *file line has it, KLEINMAN-BYLANDER. An element with no such comment uses the built-in table (see writing a CPMDParams message). An element in neither fails before CPMD starts, and the error names the atomic number. When the method deck has no &CPMD, &SYSTEM, or &DFT section at all, a minimal isolated deck replaces it: OPTIMIZE WAVEFUNCTION, CONVERGENCE ORBITALS 1.0d-5, MAXITER 40, SYMMETRY 0, POISSON SOLVER HOCKNEY, and the functional, cutoff, charge, and multiplicity of the message.
If the deck has no CELL keyword, a CELL line from ForceInput.box is inserted after ANGSTROM in &SYSTEM, or a 12 Angstrom cube when the step has no box.
If the deck has no MAXITER keyword, MAXITER 40 is inserted after &CPMD.

The method text is kept in full, including a block longer than a few thousand characters. A preview buffer shorter than the composed deck returns an error and does not keep a shortened copy.

So read method.inp with those three edits in mind. The &ATOMS block in the file lists the pseudopotential lines with a placeholder atom count of 0; the coordinates come from each step.

Checks worth making on the file¶

Look for	Why
`SYMMETRY` and `POISSON SOLVER` in `&SYSTEM`	`SYMMETRY 0` with `POISSON SOLVER HOCKNEY` is an isolated system; a message without a `system` section renders exactly that
`CELL`	present means the message fixes the cell; absent means each step’s box supplies it
`KLEINMAN-BYLANDER` on the `*file` line	on the `LMAX` line CPMD ignores it and integrates the projectors by Gauss-Hermite
`MAXITER`	absent means the OpenCPMD path caps the SCF at 40 steps
the optimiser keyword (`ODIIS`, `PCG`, `PCG MINIMIZE`)	decides the cost per SCF step; see choosing the optimiser
`FUNCTIONAL` in `&DFT`	the functional CPMD uses; the `dft` section wins over the top-level `functional`

Compare with a deck that works¶

When a hand-written CPMD deck gives the expected energy under cpmd.x and the library does not, compare the two after the edits above:

diff -u reference.inp method.inp

Differences in &ATOMS are expected: coordinates, atom counts, and the species order come from the step. Every other difference is a field that the message sets differently or leaves to a default.