Comments on the Influence of Intermediate Excitons and Exchange Forces on the Interaction in Molecular Crystals: The Crystal Structure of Chlorine

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Abstract
The reasons for the failure of the Lennard‐Jones potential to predict the correct space group of solid chlorine and the molecular orientation within the unit cell are examined. In the analysis given, the contribution of charge‐transfer states to the intermolecular binding energy in the ground state is calculated. It is found that these states account for about 25% of the intermolecular interaction, and tend to reinforce the dispersive—repulsive forces in the region of the observed molecular angle of tilt. However, in our approximation these forces fail to account for the observed stability of the orthorhombic structure relative to the cubic structure, the latter being predicted to be more stable if the interaction is a Lennard‐Jones potential. The repulsive interactions are calculated using an approximation to the exchange energy. It is concluded that the failure of the Lennard‐Jones potential to predict the correct angle of molecular tilt within the unit cell arises from the incorrect form of the repulsive potential. Further, with the binding energies calculated in this paper, the difference between the energies of the cubic and orthorhombic structures is much reduced from that predicted from calculations based on the use of the Lennard‐Jones potential.

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