8.2 Car–Parrinello dynamics: keep fictitious electrons adiabatic

These are unexecuted teaching inputs and starting models. Original diagrams are schematics, not calculated results. Validate version-specific syntax, licensed or authorized data, numerical convergence and the scientific model before using this workflow.

8.2.1 Model, units and provenance

PW cutoffs and energies use Ry, common force output uses Ry/bohr, and pressure uses kbar. Geometry cards state their coordinate units. Different executables have distinct grammars and time-unit conventions.

Shared inputs, conventions and evidence

Original schematic: Car–Parrinello dynamics: keep fictitious electrons adiabatic. No numerical results are claimed.

8.2.2 Unexecuted inputs and explicit deltas

Use the accompanying instructions to identify the parent calculation and placement of every delta; a snippet is not automatically a standalone input. Preserve all blank-line and file-provenance requirements.

8.2.2.1 Input block 1

! Conceptual CP-specific preparation deck delta, NOT a PW input.
! Supply a CP-compatible tested SYSTEM, species, cell and positions.
&CONTROL
 calculation='cp', restart_mode='from_scratch',
 prefix='water_cp', outdir='./scratch/water_cp', pseudo_dir='./pseudo',
 nstep=200, dt=3.0, ndw=51
/
&ELECTRONS
 electron_dynamics='damp', emass=300.0
/
&IONS
 ion_dynamics='none'
/

8.2.2.2 Input block 2

! Production continuation only after successful electronic preparation.
! CONTROL: restart_mode='restart', ndr=51, ndw=52, dt=3.0
&ELECTRONS
 electron_dynamics='verlet', emass=300.0
/
&IONS
 ion_dynamics='verlet'
/

8.2.3 Worked investigation

8.2.3.1 Intuition and prerequisites

cp.x propagates electronic degrees of freedom with a fictitious mass in the Car–Parrinello formulation. This is conceptually different from converging electrons at every PW MD step. A small gapped system, carefully prepared wavefunctions and separation of ionic/electronic timescales are prerequisites. Do not rename a PW save directory and expect a CP restart; use CP's own initialization and restart format.

8.2.3.2 Original two-stage CP delta

See input block 1 above.

8.2.3.3 Checks, pitfalls and exercise

Track fictitious electronic kinetic energy, ionic kinetic energy and the appropriate conserved CP quantity. Stable ionic temperature alone does not establish adiabatic separation. Vary fictitious mass and time step together in a controlled study, compare with short BOMD forces/trajectories and inspect whether electronic energy steadily heats. For metals or small-gap systems, separation can be difficult; do not advertise CP as a universal faster substitute for BOMD. Preserve ndr/ndw lineage and avoid overwriting the only restart.

Exercise: explain why lowering fictitious mass often requires a smaller step and why fictitious electronic kinetic energy is not a physical electronic temperature. Prepare a CP/BOMD comparison sheet with physical duration, force accuracy, conserved quantity and computational cost, leaving entries blank until both methods have actually run.