Goal

Pick the CPMD wavefunction optimiser for a session that evaluates many nearby geometries, such as a minimisation, a saddle search, or a nudged elastic band. Every call after the first starts from the orbitals of the previous call, so the optimiser that suits a cold start from atomic orbitals is not always the cheapest for the warm calls that follow.

Measured on the Si3N4 cluster

One minimisation of a 7-atom Si3N4 cluster, BLYP at 70 Ry in a 14 Angstrom cube with Kleinman-Bylander pseudopotentials, library route on 16 MPI ranks, orbital convergence 1e-5. The two runs differ only in the optimiser line of the &CPMD section.

Optimiser	Wall time	Force calls	SCF steps	Seconds per SCF step	Minimum (eV)
PCG MINIMIZE (the deck’s choice)	734 s	33	515	1.26	-1396.271181
ODIIS with 10 vectors	255 s	23	296	0.64	-1396.271246

Both reach the same minimum: the two energies differ by 0.065 meV, inside the orbital convergence tolerance. ODIIS (orbital direct inversion in the iterative subspace) needed fewer and cheaper SCF steps. PCG (preconditioned conjugate gradient) with MINIMIZE runs a line minimisation that costs about two energy evaluations per step.

Set ODIIS in the message

( cpmd = ( optimizeWavefunction = true,
           convergenceOrbitals = 1.0e-5,
           maxIter = 400,
           odiisVectors = 10 ) )

odiisVectors = 10 renders ODIIS followed by 10 on the next line. With no optimiser keyword at all, OpenCPMD 4.3 also uses preconditioned DIIS with 10 vectors; its output then reads WAVEFUNCTION OPTIMIZATION BY PRECONDITIONED DIIS and MAXIMUM NUMBER OF VECTORS RETAINED FOR DIIS: 10. Writing odiisVectors keeps the choice visible in the message. odiis = true renders a bare ODIIS with CPMD’s default vector count, and odiisOptions adds inline options such as NOPREC. To keep the deck’s PCG line instead, use a directive:

( cpmd = ( optimizeWavefunction = true,
           convergenceOrbitals = 1.0e-5,
           maxIter = 400,
           directives = [ ( keyword = "PCG MINIMIZE", args = [] ) ] ) )

Set only one optimiser in a message. Check the rendered &CPMD section with CPMDC_DECK_OUT (see debugging a deck).

When to keep PCG

cpmdc applies one deck to every call of a session, so the optimiser chosen for the warm calls also runs the cold first call from atomic orbitals. Keep PCG MINIMIZE for a system whose ODIIS calls stop at MAXITER: each such call is reported as a failure (see troubleshooting), and the optimiser choice does not change the converged state, only the path to it.

Always set MAXITER

Without maxIter, the OpenCPMD path inserts MAXITER 40 into the first deck. A warm call on a geometry that moved far from the previous one can need more steps than that, and an SCF that stops at the limit fails the call.