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.