An account is presented of our studies of the order-disorder phase transitions in NaCN and KCN. These are based on parameter-free inter-ionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions. We performed static structural relaxations and supercell molecular dynamics and reproduced two transitions known in each of these systems. We also calculated upper bounds to the barrier of rotation of a cyanide ion in a ground state of NaCN and KCN and discuss possible shortcomings of our model. ©2000 American Institute of Physics.