Lattice dynamics. Phonon frequencies are useful to us for three reasons. First, that all frequencies are real confirms that the structure corresponds to an energy minimum. Secondly, they allow us to calculate the vibrational entropic contribution to the binding energy of the guests. And finally, phonon frequencies can be compared to experimental Terahertz or low-frequency Raman spectra, possibly allowing structural determination by direct comparison to predicted frequencies . Rigid-molecule geometry optimizations and lattice dynamics calculations, using the above force field, were performed with DMACRYS .