Metadynamics is known to work especially well in terms of computational time and accuracy when the free energy surface is represented in a space of two or more collective variables. Since we were interested in the dissociation of ammonium as well as the likelihood of the nitrate and the ammonia/ammonium pairing together throughout the trajectory, we used the following two collective variables: (i) the distance between the nitrogen atoms of the nitrate, and (ii) ammonium and the coordination number of the nitrogen and bonded hydrogen atoms in the ammonium molecule. The coordination number between atoms A (the nitrogen of the ammonium) and atoms B (the hydrogens of the ammonium) is defined as: