Recently, I have come across the MFP/TIP3P water model. The authors of the paper (https://pubs.acs.org/doi/10.1021/ct300011h) which describes this model states that it is implemented in GROMACS-4.0.7 and they refer to GROMACS-manual-4.0. But unfortunately, I am unable to find any documentation (including the manual) which describes how to use MFP/TIP3P water model with GROMACS. But, I could find many papers where this model has been used with GROMACS-4.0.7, but nowhere it is written how to use it.
This looks like a water model that required some changes to the source code (in contrast to merely adding a parameter file) It seems to me that the authors implemented their own changes to the GROMACS code that for multiple reasons have not been incorporated into an official version of GROMACS (most likely, because the authors never made an attempt at this).
Asking the authors directly might be the best way forward if you want to use their water model.