The next step in the development of PES-2023 was the fitting process of the 66 parameters to the input data. In 2009, our group 26 developed a least-square method, which uses energies, gradients and Hessians to attempt to automatize the fitting process and to obtain the best set of parameters. However, when all parameters are included, the automatic process is unsuccessful, and finally a trial-and-error hand-made process is needed. Thus, the fitting process is divided in four iterative steps. The description of the reactant and product properties, such as geometry, vibrational frequencies and energy of reaction, represents the first, and simplest, step. For instance, the reaction energy is related with the difference of the dissociation energies for the formed and broken bonds, which are two adjustable parameters. The description of the saddle point is a delicate issue, and represents the second step. Here, we should pay special attention to the barrier height and the imaginary vibrational frequency, which is related with the curvature of the path in this stationary point. The following step is the description of the reaction path from reactants to products. Here, the falls to reactants (smooth) and products (pronounced) from the saddle point are simulated. This is a complicated and laborious process because of the asymmetry of the reactant and product valleys, which involves many parameters. Finally, the fourth step is the description of the intermediate complex properties in the entrance and exit channels. In the present reaction, this step represented a laborious task (specially the location of the complex in the entrance channel) because of the different stability of both complexes. Once these four steps were performed, we obtained the first iteration and the first set of the 66 adjustable parameters, which represents the first approximation to the problem. These steps must be iteratively repeated until a reasonable agreement with the input data is reached, obtaining thus the final set of parameters. Note that the process is tedious and a time-consuming task. In sum, the new surface, named PES-2023, presents a physically intuitive functional form (related to familiar nuclei motions), the first energy derivatives (gradients) are numerical, it is symmetric with respect to the exchange of the six hydrogen atoms in ethane, and finally, it depends on a reduced number of adjustable parameters. The new surface will be published in the online version of POTLIB. This section finishes with a comparison with the input data, because the accuracy of a PES is measured by its ability to simulate the ab initio information used as input data, which is, basically, a self-consistency test. Figure and previous theoretical calculations (1.797/1.133 Å). In this case, with respect to the reactants and products, the broken and formed bonds increase by 2 and 67% respectively, indicating a clear "early" transition state, i.e. it appears soon on the reaction coordinate, in the entrance channel, which is related with a very exothermic reaction. This behaviour has, obviously, dynamics consequences, as will be analysed later. The corresponding imaginary vibrational frequency, related with the low barrier height and associated with the curvature of the reaction path, is expected to be very low. The PES-2023 frequency is 150 i cm -1 , simulating the ab initio information, 196 i cm -1 , and another theoretical result, 212 i cm -1 , related in that study with a higher barrier, 0.41 kcal mol -1 . Finally, PES-2023 correctly describes the presence of intermediate complexes in the entrance and exit channels. The reactant complex is stabilized by 0.27 kcal mol -1 with respect to the reactants, and the corresponding product complex by 1.10 kcal mol -1 with respect to the products. These stabilities are slightly lower than those obtained with ab initio results.