Molecular dynamics simulation of crystalline naphthalene

A realistic molecular dynamics calculation for naphthalene is presented and a comparison is made between the simulation and the crystallographic data. The molecular dynamics sample consists of 4096 molecules arranged in a volume with cyclic boundary conditions, each molecule being associated with one processing element of the ICL DAP computer. The potential functions used are the same as those used for lattice dynamics, and are developed from atom- atom 6-exp functions. A comparison with a lattice dynamical result establishes the correctness of the program. Temperature is introduced in the usual molecular dynamics way, resulting in a true modelling of anharmonic behaviour. The calculation proceeds at zero pressure throughout, yielding the temperature variation of the crystalline unit cell, the mean molecular orientation and the rigid-body thermal vibration tensors T and L. These are compared with the recently reported results measured by neutron scattering from powder samples and analysed through constrained refinements. The variation of unit-cell volume with temperature is particularly close to the experimental result, though discrepancies begin to be significant in measurements involving the orientational behaviour of the molecules.