guidelines on implementing special kind of collective variable

[Peter]

Dear CP2K developers!

I would like to try to implement a special type of collective
variable: it is a difference between potential energy of a system and
potential energy of the system in the same geometry with an extra
electron. In other words, I want to use vertical ionization energy as
a collective variable for e.g. metadynamics.
If there are any caveats regarding this task? E.g. with potential
derivatives calculation.
Could you please give some guidelines on implementing this in cp2k?

Thanks,
Peter