Dear, all. As the title shows, my model uses PCFF force field, but the added water molecules want to use SPC model. The SPC model uses 12-6 LJ potential energy, while the PCFF force field uses 9-6 LJ potential energy. I don’t know how to combine them. Or is there a water molecular model more suitable for PCFF force field？
Technically, you could do that using pair style hybrid. But that is not a good idea. Changing the water potential requires changing the charges, but that would unbalance the interactions of water with all the other atoms unless you reparameterize them. At that point it would just be as easy as creating a new complete force field.
There are water parameters that are part of the force field. That is what the force field was parameterized with in general and thus that is what you should be using. It is not considered a good idea to change only a part of the force field, as that would invalidate all other parameters.
That said, you can certainly search the published literature for people that have studied systems using PCFF that included water and check what water potential those used.