Calculation of thermodynamic properties using the random-phase approximation: alpha-N2

The random-phase approximation (RPA) for molecular crystals is extended in order to calculate thermodynamic properties. A recursion formula for thermodynamic averages of products of mean-field excitation and deexcitation operators is derived. With this formula the thermodynamic average of any operator can be obtained in the RPA by writing the operator in terms of mean-field (de)excitation operators. The energy of a-N2 is thus calculated as a function of pressure and temperature. The specific heat, free energy, cell parameter, isothermal compressibility, and expansion coefficient are derived from the energy. The elastic constants and the Grüneisen parameters of a-N2 are also given.