Atomistic calculations of phonon frequencies and thermodynamic quantities for crystals of rigid organic molecules

Rigid-body, k = 0 phonon frequencies have been calculated within the crystal structure modeling program DMAREL, enabling the use of anisotropic atom-atom model potentials. Five organic crystals (hexamethylenetetramine, naphthalene, pyrazine, imidazole, and alpha-glycine) were chosen to sample a range of intermolecular interactions for determining the sensitivity of the calculated frequencies to changes in the empirical repulsion-dispersion parameters and the electrostatic model. A carefully parameterized simple exp-6 model can describe vibrations in simple van der Waals crystals and some hydrogen bonded crystals reasonably well. However, for weaker polar interactions, an accurate model of the electrostatics is needed. Bending of weak polar interactions and shearing of close contacts with delocalized pi-systems are particularly sensitive to the description of electrostatic interactions. Point charge models generally underestimate the resistance to deforming hydrogen bonds, and a distributed multipole model stabilizes these interactions. Because of their statistical nature, vibrational contributions to the energy can be estimated more accurately than the frequencies of individual modes, and the best models give good estimates of zero-point energies and the vibrational partition function, which should be useful in predicting the relative stability of polymorphs.