Structural disturbance and lattice relaxation in molecular crystal packing calculations

Lattice calculations on impurity-containing crystals are reported, made with the help of standard intermolecular potentials. A lattice containing a substitutional impurity, or a vacancy, relaxes in its vicinity, the equilibrium structure being little changed in either orientation or translation. The small changes do however enable the strain energy to be greatly reduced. The energies are sensitive to the assumed intermolecular potentials but the optimised structures are rather insensitive. An impurity larger than the host (tetracene in anthracene) adopts closely the host orientation. This applies also to a smaller impurity (e.g. napthalene in anthracene) but in this case the impurity may move away from the lattice site to one of two inversion related displaced positions. The cage surrounding a vacant site relaxes little, leaving the “hole” more or less intact. Lattice relaxations found in this way seem too small to accommodate guest—host photochemical reactions in those lattices which, in the pure crystal, are photochemically stable.