Jean-Lois Rivail and co-workers have developed a calculation method for large systems, the so called Hybrid Classical Quantum Force Field, which uses a local self consistent field approach (LSCF) with neglect of diatomic differential overlab integrals to optain continuity in the link region between a QM-subsystem (S) and a big classical system (the rest of the molecule) (E) [Ferenczy et al., 1992] [Monard et al., 1996]. One hybrid orbital of the frontier atom (the one linked to the E-system) is frozen while an orthogonal basis is created from te rest of the hybrid atomic orbitals, and this basis is used to calculate a self consistent field. Thus the method introduces a new Hartree Fock equation which is implemented through gathering of the AMBER and GEOMOP programs. The method is tested on various systems, e.g. oligopeptides [Théry et al., 1994]. The method has also been expanded by the group to the ab initio level [Assfeld and Rivail, 1996], and it has been used to describe chemical reactions, for example proton exchange processes [Reuter et al., 1997] and hydrolysis by the enzyme thermolysin [Antonczak et al., 1998].