In general, all development of PESs begins with an extensive number of electronic structure calculations describing the reactive system, which represents the input data. In the VB-MM framework, the process continues with the proposition of different functional forms describing typical nuclei motions in the reactive system, stretching, bending in the plane and out-of-plane, torsions etc., which depend on a set of adjustable parameters. The process finishes with the fitting of these parameters to the input data.