Based on the experience from our research group (Refs. 9, 10 and references therein), the development of a new PES is a three-step process, which begins with the input data, i.e., an extensive number of electronic structure calculations describing the whole reactive system. The process continues with the proposition of a functional form, which describes motions of stretching, bending in the plane and out-of-plane, torsions etc., of the reactive system. To make this functional more flexible, it depends on a set of adjustable parameters. Based on our experience, the new PES is developed in the VB-MM framework, valence bond-molecular mechanics. The third step is the fitting of these parameters to the input data.