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vibrations

vibrations <key1> <args1> ...

Calculate vibrational modes of the system using a finite difference method. Note that this command should typically be issued in a run with converged ionic positions; ionic (and lattice) minimization are bypassed by the vibrations module.

Any number of the following subcommands and their arguments may follow:

- dr <dr>: perturbation amplitude in bohrs for force matrix calculation (default: 0.01).
- centralDiff yes|no: use a central difference formula for the second derivative to achieve higher accuracy at twice the cost (default: no)
- useConstraints yes|no: restrict modes of motion as specified by move flags and constraints in the ion command (default: no)
- translationSym yes|no: whether to assume overall translation symmetry (default yes). Can be turned off to get vibrational levels in an external potential.
- rotationSym yes|no: project out rotational modes (default no). Improves reliability for molecular calculations. Valid only for geometries with an unambiguous center of mass.
- omegaMin <omegaMin>: frequency cutoff (in Eh) for free energy calculation (default: 2e-4)
- T <T>: temperature (in Kelvin) for free energy calculation (default: 298)
- omegaResolution <omegaResolution>: resolution for detecting and reporting degeneracies in modes (default: 1e-4). Does not affect free energies and all modes are still printed.

Note that for a periodic system with k-points, wave functions may be incompatible with and without the vibrations command due to symmetry-breaking by the perturbations. To avoid this, perform electronic optimization (without initial-state) in the vibration calculation itself, or consider using the phonon code instead.

**Requires:** (None)

**Forbids:** fix-electron-density fix-electron-potential

**Allow multiple:** no

**Default:** (None)

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