The thermodynamic stability of the low temperature alpha, gamma, and epsilon phases of nitrogen is examined using an energy minimization technique. Three of the most successful potential forms available are studied and the results are not satisfactory. A recent potential form is modified by optimizing its parameters to seek agreement with experiment. This search yields phase transitions between the alpha, gamma, and epsilon phases that are in good agreement with experiment, as are other important thermodynamic properties. The reason for this success is explained. (C) 2000 American Institute of Physics. [S0021-9606(00)51315-4].