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The effects of the zero-point energy, residual forces and torques on the molecules, and molar density, on the calculated lattice frequencies and structural properties of solid α- and γ-N2, are investigated. An intramolecular potential and a parametrized 12-6 atom–atom intermolecular potential are used to calculate lattice modes, intramolecular modes, sublimation energy, equilibrium unit cell parameters, Grüneizen coefficients, and P–V data. The second virial coefficient is also reasonably reproduced. The α-to-γ phase transition is not revealed by the employed intermolecular potential.

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A general method for the simultaneous calculation of the effect of inter-and intramolecular forces on crystal packing and lattice dynamics is described. The crystal energy is analytically represented as a function of the 3n Cartesian coordinates of the n atoms in the asymmetric unit and the 6 unit-cell parameters. The equilibrium geometry and the lattice vibrations are consistently derived from the same crystal potential. The applicability of the method for both rigid and flexible molecules is demonstrated by calculations of benzene, biphenyl and fl-ionylidenecrotonic acid. The computer programs developed in this work are available.

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