Lattice dynamical calculations of the mean square amplitudes of crystalline biphenyl

Abstract

The molecular mean-square-amplitude matrix of crystalline biphenyl, C 12H 10, is calculated with an intermolecular potential of the atom -atom type. The effect of the presence of the low-lying torsional mode, which interacts with translational modes, is discussed. The Born S-matrix method is used, and proves to be an excellent approximation, which takes about j of the computer time of the exact calculation. The resulting amplitudes are in fair agreement with experiment, and show that the extremely high amplitude of libration about the long molecular axis may be satisfactorily explained without assuming a double-well shape for the torsional potential.