Knowledge Bank

A search for consistent intermolecular potential functions in molecular crystals was initiated with a study of hexachlorobenzene (HCB) and solid chlorine. The potential model was based on atom-atom interactions including repulsion, Van der Waals ‘ attraction, and Coulomb terms. For a distance $rij$, between non-bonded atom pairs, the potential function given by $$v(r_{ij}) = \frac{q_{i}q_{j}}{r_{ij}} - \epsilon_{ij} \left[ 6 \left(\frac{B_{i} + B_{j}}{c_{ij}}\right) -1\right]^{-1}\left[-\left(\frac{\sigma_{ij}}{r_{ij}}\right)^{6}+6 \left(\frac{B_{i} + B_{j}}{\sigma_{ij}}\right) exp \left( \frac{\sigma_{ij} - r_{ij}}{B_{i} + B_{j}}\right) \right]$$ was found to give the best agreement between observed and calculated properties. It was assumed that the molecular solids were composed of rigid bodies, and the energy parameters in the above equation were varied to achieve the best agreement between the observed and calculated crystal structure, lattice energy, and k = 0 external mode frequencies. For both HCB and solid $Cl2$, it was found that, although the contribution to the lattice energy from coulomb interaction was small, these terms were essential to achieve minimum energy configurations which agreed with the experimental crystal structures.