FCFit program

FCfit is program for the simulation and fit of vibronic absorption and emission spectra based on the evaluation of relative Franck-Condon (FC) factors and/or Franck-Condon-Herzberg-Teller (FCHT) theory. The program computes the FC integrals of multidimensional, harmonic oscillators mainly based on the recursion formula given in the papers of Doktorov, Malkin, and Man’ko. The fit of vibronic spectra can greatly be improved if independent information for the geometry change upon electronic excitation is available. This information can be the change of the rotational constants upon electronic excitation. While geometry fits to the rotational constants alone are possible and routinely made, using non-linear fits in internal coordinates as basis for the geometries, the combination of rotational constant changes and vibronic intensities allows for determination of much more geometry parameters. In contrast to the separate geometry fits of the two states using rotational constants, in the combined FC fit geometry changes (relative to one of the states, usually the ground state) are determined in the basis of selected normal modes.