We present interatomic pair potential parameters derived for the GaInAlN system. Potentials are fitted to bulk material properties, such as lattice constants and elastic and dielectric constants, and are then employed to calculate Schottky and Frenkel defect energies in GaN, InN and AlN. Schottky defects are found to be lower in energy than Frenkel defects, suggesting that vacancies are more readily formed in the group-III nitrides. The formation energies of both Schottky and Frenkel defects in all the nitrides are found to depend on the size of the cation. Solution energies indicate that InN readily dissolves in both GaN and AlN, at least at low concentration.