The gas-phase protonation mechanism of ferrocene is investigated using an effective potential describing electrostatic, polarization and short-range repulsion energy components. The parameters of the potential are deduced from a least-squares fit to energy profiles calculated using local-spin density or Hartree—Fock methods. It is shown that this model, which provides a clear physical interpretation, leads to a good description of this reaction mechanism. In addition, a molecular dynamics simulation of ferrocene based on an intramolecular force field developed in this work is presented.