Empirical atom-atom potential for a naphthalene crystal and transferability to other polyacene crystals

Intermolecular potentials for polyacenes are represented by an atom-atom potential approximation using a revised carbon-carbon interatomic potential. The potential has an attraction term with an r⁻ⁿ(n = 6, 8, or 10) dependence and a repulsion term of exp (- Cr) dependence with respect to the interatomic distance r; parameters were optimized for the crystal structure and the heat of sublimation of naphthalene by molecular dynamics simulations. The potential shape is deeper and steeper at short ranges than generally accepted former potentials. Crystal structures of polyacenes (benzene, naphthalene, anthracene, tetracene, pentacene, and hexacene) are calculated numerically by molecular dynamics at room temperature using the potentials. In accordance with experimental results, the calculated crystal systems and space groups are orthorhombic Pbca in benzene, monoclinic P21/a in naphthalene and anthracene, and triclinic PI in tetracene, pentacene, and hexacene for all n, except benzene for n = 10. The agreement in the lattice constants and molecular orientations is fairly good and best for n = 8: The average deviations are 0·8% and 0·8° for the unit cell lengths and angles, respectively, and 2·0° for the molecular orientation.