To explore efficiently the free energy surface of ammonium nitrate deposited on ice, and investigate the likelihood of ion pairing and deprotonation of ammonium, we employed welltempered metadynamics. Metadynamics is an enhanced sampling method where a historydependent bias potential V (s) is constructed on the fly as a function of the atomic trajectory via a restricted number of collective variables (CVs), s = s(R) expressed as a function of the coordinates of the system (R). Collective variables are low dimensional descriptors of molecular structure. The fluctuations on the CVs can be gradually enhanced, allowing for large energy barriers to be overcome and rare events to take place in reasonable computational time. In the well-tempered version a Gaussian G(s, s k ) centered at a visited point (s k )