The `FCON` program allows for the calculation of Franck-Condon factors based on potential energy surfaces obtained from
the `SURF` program and vibrational wavefunctions as provided by the `VSCF` or `VCI` programs. Duschinsky effects may or
may not be included. These can either be applied to the vibrational wavefunction (of the vibrational ground state) or the potential
by using the `PESTRANS` program. The latter possibility is the recommended one as it is significantly faster.
The `FCON` program including Duschinsky rotations can only be used with analytical representations of the
potential energy surfaces.
A prescreening of the Franck-Condon factors without Duschinsky effects at the `VSCF` level is used to reduce the computational effort for
correlated levels, e.g. `VCI`. Note that, Franck-Condon factors at the uncorrelated `VSCF` level including Duschinsky effects are
usually of fairly poor quality. As the calculation of Franck-Condon factors often involves very high quantum numbers for the vibrational
states of the final electronic state, very high excitation levels must be enabled in the `VCI` calculations, i.e. see keyword
`LEVEX`. As a consequence, the `SCALE` parameter in `SURF` calculations needs to be modified in most applications.