Lattice dynamics of iodine

Calculations of lattice vibrations for crystalline iodine show that the Buckingham-type interaction is inadequate to explain the low-frequency spectra. A charge-transfer term is included in the intermolecular potential to reproduce an electronic interaction. Thus external frequencies calculated with this modified potential fit the spectroscopic data well. Dispersion curves of external and internal modes of vibration are also calculated and compared with N.I.S. measurements. We have also calculated the one-phonon density of states and deduced the heat capacity of iodine at low temperatures. We also discuss the criteria used to verify the classical harmonic treatment of the iodine lattice librations.