A simplified force field for general application to aromatic polycyclic hydrocarbons concerning ‘out-of-plane’ vibration is presented and discussed. A discriminating fit to frequencies obtained from the vapour phase and the crystal is relevant in this case, and harmonic lattice dynamical calculations have been applied for this purpose. On the whole, satisfactory results can be obtained even from very simple fields, involving two to five torsional coordinates, with no interaction constants: for each molecule, the force constants can be evaluated from geometrical considerations and Kekule structures. Examples of application to isolated molecules and crystals of benzene, naphthalene, anthracene, phenanthrene and pyrene, including some deuterium analogues, are given: the average disagreement between observed and calculated frequencies is 13 cm-1 for the five parameter field.