index | modules | gitlab | page source

# Evaluation of Franck-Condon factors

The Franck-Condon principle couples electronic and vibrational properties.

## Franck-Condon factors in CH4

### Forces

We may get a good structure for methane from `ase.build`. This will not be the ground state structure according to the `EMT` calcuator though. Therfore `EMT` predicts finite forces:

    from ase.build import molecule
    from ase.calculators.emt import EMT

    atoms = molecule('CH4')
    atoms.calc = EMT()

    # evaluate forces in this configuration
    forces_a = atoms.get_forces()

### Vibrational properties

These forces can be used to calculate the corresponding Franck-Condon factors for transitions from the `EMT` ground state. First we need the `EMT` ground state and the vibrational properties:

    from ase.optimize import BFGS
    from ase.vibrations import Vibrations

    # relax and get vibrational properties
    opt = BFGS(atoms, logfile=None)
    opt.run(fmax=0.01)

    vibname = 'vib'
    vib = Vibrations(atoms, name=vibname)
    vib.run()
    vib.summary()

### Huang-Rhys factors

The Huang-Rhys factors describe the displacement energy in each vibrational coordinate relative to the vibrational energy. We may get them by:

    from ase.vibrations.franck_condon import FranckCondon

    # FC factor for all frequencies
    fc = FranckCondon(atoms, vibname)

    HR_a, freq_a = fc.get_Huang_Rhys_factors(forces_a)

### Franck-Condon factors

The Franck-Condon factors depend on temperature due to occupation of vibrational states. We may get them for 293 K by:

    FC, freq = fc.get_Franck_Condon_factors(293, forces_a)

where `FC[0]` contains the Franck-Condon factors and `freq[0]` the corresponding frequencies.

It is also possible to evaluate higher order transitions. Two vibrational quanta might be considered by increasing the order:

    FC, freq = fc.get_Franck_Condon_factors(293, forces_a, order=2)