For a Plumed nest archive with example input files for a path-metadynamics simulation of acetic acid dissociation and proton transfer with Plumed and CP2K click on this link. Further instructions: These CP2K/Plumed input files are for a PMD simulatation of the acetic acid de-protonation reaction in water and pKa calculation as performed in Ref. [2]. It requires PLUMED compiled with the Path-CV code (PathCV.cpp provided below) and the PTCV code (proton.cpp provided within this archive). All the parameters used are explained in the plumed input file and the article in Ref. [2].
To add the PROTON (proton transfer collective variable) functionality to your plumed, copy the Proton.cpp file found in the above Plumed nest archive in your plumed-2 distribution in the directory src/vatom/Proton.cpp. Then recompile the Plumed code (make, make install, make doc). The documentation on how to use the proton transfer CV is found in user-doc/html/index.html under Collective variables / Groups and Virtual Atoms / PROTON.