pka from molecular dynamics How can one calculate pKa values from first principle calculations, static or dynamics?

I have encountered some works using CPMD where people try to find pKa values from -(i) constrained Molecular Dynamics (MD) and (ii) metadynamics. These are somewhat more intricate ways, I believe. Is there any other simpler ways (theoretical/computational)? Can not we do it from simple MD? 
Please suggest some references as well, if possible.

Thanks in advance.
Have you considered a free-energy calculation?

That is, calculate the free-energy of the dissociated species and of the neutral one, and calculate the pKa via the formula for activity.

There was this GROMACS tutorial, but it seems the link is broken.