Application of semiempirical atom–atom potentials to crystals of acetylene

Semiempirical atom–atom potentials of the ’6-exp’ and ’Lennard-Jones’ type have been applied to crystals of acetylene, in order to account for various experimental data, such as sublimation energy, unit cell parameters, lattice vibration frequencies, etc. Most potentials which are generally satisfactory for hydrocarbons, do not account for the lattice vibration frequencies in acetylene: for the cubic phase, a good fit to several experimental data can be obtained using appropriate functions, provided that the sublimation energy is substantially lowered with respect to the experimental value. However, no potentials of this kind, at least in the very extensive range so far tried, can account for the properties and the very existence of the orthorhombic phase, as it is described in the literature. Before pointing to an inadequacy of the present form of these potentials, there is anyway the need of remeasuring the sublimation energy of acetylene, as well as collecting and refining single crystal data also for the orthorhombic phase.