MODELLING THE THERMAL MOTION OF FREELY ROTATING MOLECULES. R. T. Downs and L. W. Finger, Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. N.W., Washington D.C. 20015, phone: (202) 686-2410 x 2469, e-mail: downs@gl.ciw.edu

All molecular gas-phases become crystalline when subjected to sufficient pressure. Spectroscopic studies show that the molecules are freely rotating while near the freezing point for many of these phases. As pressure increases the motion is constrained and a lowering of symmetry occurs. In order to model the freely rotating molecule we calculated molecular electron-density wave functions using the Gaussian92 program for energy-optimized model molecules. The electron-density was then spherically averaged in order to mimic the rotations. The Fourier transform of this spherically averaged electron density provides the scattering factor for the rotating molecule. The displacement parameters obtained from the structure refinement then provide estimates of translational motion.