In this chapter we shall be concerned once again with the very basis of the CFF concept. It will be recalled that this concept way be formulated as: choice of analytical forms of potential energy functions for a faaily of molecules; selection of initial values of the parameters of the energy functions; calculation of any desired observable property of each wolecule; optimisation of the energy parameters to provide a better fit between calculated and weasured observables. We shall treat here this concluding step.