Intermolecular Forces and the Space Group of Solid Chlorine

Use of the 6:12 potential for Cl₂, and presumably for other halogen molecules, shows that the observed orthorhombic Cmca space group in which they crystallize is not that predicted; both the cubic Pa3 and the hexagonal $\text{P6̄m2}$ would be more stable crystal structures. A molecular model is discussed in which, compared to the free halogen atoms, the molecule has, in addition to concentration of charge at its center, a charge concentration at its outer ends. This model correctly predicts the Cmca space group and the orientation of the molecules in the unit cell. The calculations also show why certain other simple molecules crystallize in Pa3.