PERTURBATION-THEORY FOR THE FREE-ENERGY OF HARD QUADRUPOLAR DIATOMICS

The perturbation theory for the free energy of hard spheres with an embedded quadrupole is extended to molecules with a nonspherical core. Our system consists of quadrupolar hard homonuclear diatomics. The free energy is calculated to order Q6 in the quadrupole strength. Several expressions for hard diatomics are examined to obtain the free energy of the unperturbed system. The term of order Q2 is calculated using recent Monte Carlo results for the reference system and it is found that it gives an important contribution to the total free energy. The Padé approximant (PA) formed for the quadrupolar hard spheres with the terms of order Q 4 and Q6 is evaluated with an effective diameter calculated analytically using the isothermal compressibility of the hard diatomics. It is concluded that it is necessary to take into account the nonsphericity of the molecules in the calculation of the free energy in order to obtain reliable results for real systems. © 1979 American Institute of Physics.