This part is organized as follows. The first subsection presents the results of the model optimized to reproduce the experimental RDFs (g OO , g OH , and g HH ) under standard conditions of 298 K and 1 bar. This approach focuses primarily on a pure bottom-up methodology, where the optimization is driven by the microscopic features of the water model. In the second subsection, we extend our analysis by optimizing the model to reproduce not only the RDFs but also experimental density and static dielectric constant. Furthermore, the system is trained at two additional temperatures, specifically 280 K and 343 K. This comprehensive optimization approach aims to capture a broader range of experimental observables, combining both bottom-up and top-down references. Finally, the last subsection provides a detailed investigation into the indeterminacy of the optimization problem within the context of the three-site representation.