  Acid-base reactions are ubiquitous in nature. Understanding their mechanisms
is crucial in many fields, from biochemistry to industrial catalysis.
Unfortunately, experiments only give limited information without much insight
into the molecular behaviour. Atomistic simulations could complement
experiments and shed precious light on microscopic mechanisms. The large free
energy barriers connected to proton dissociation however make the use of
enhanced sampling methods mandatory. Here we perform an ab initio molecular
dynamics (MD) simulation and enhance sampling with the help of methadynamics.
This has been made possible by the introduction of novel descriptors or
collective variables (CVs) that are based on a conceptually new outlook on
acid-base equilibria. We test successfully our approach on three different
aqueous solutions of acetic acid, ammonia, and bicarbonate. These are
representative of acid, basic, and amphoteric behaviour.
