Lattice dynamics of solid I2 under high pressure

A calculation of the lattice dynamics of solid I2 is made on the basis of a semiphenomenological potential with terms describing the intramolecular interaction, orientation-dependent charge transfer interactions, and nonbonding interactions. In addition, two terms describing a coupling between intramolecular vibrations of different molecules and a vibrational coupling between charge transferred bonds are included. At normal pressure, calculated vibrational frequencies are in good agreement with Raman and neutron scattering data. Agreement with experiment at high pressures requires that some potential parameters be volume dependent, and the implication of this result to the pressure induced change in the electronic states of solid I2 is discussed. The merging of the intramolecular Ag and B3g Raman bands upon compression is explained quantitatively. The pressure dependence of several lattice vibrational mode frequencies are predicted but the observed softening of the librational Ag mode is not reproduced. The crystal energy is also calculated and compared with experiment.