The energy for the hollow-adsorbed NH is much higher than the NH2 reconstructions as well as both the hollow and bridging N2H2 dimer. The large free energy difference between the NH adsorbed at a hollow site and the N2H2 dimers, around 100 kJ/mol, shows that the dimer is much preferred compared to the monomer at STP due the very strained geometry of the NH(ho) monomer. For only nitrogen the preferred reconstruction on AlN and GaN is the formation of dimers, with adsorption free energy much lower than an adsorbed N atom due to the strained structure of the monomer and presence of unpaired electrons on the lone N atom.