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As a part of a systematic project of evaluating temperature factors for molecular crystals in a harmonic approximation, following the Born-von Kàrmàn procedure, application to essentially 'rigid' aromatic hydrocarbons has been reconsidered. In this approximation, where 'rigidity' just implies non-mixing between internal vibrational modes and external ('lattice') modes, contributions of the former to the temperature factors of the various atoms have been added to the corresponding contributions of the latter modes. Following Gwinn's method for normal coordinate analysis in the molecule, and Pawley's method for calculating the lattice modes, it is possible to write highly efficient routines, so that practical application to the various molecules becomes easy and straightforward, the final result being reached in a quite reasonable computing time, with a 'usual' crystallographic input. For intermolecular interactions, Williams IVa potentials have been used, and a Califano-Neto harmonic force field has been adopted for calculating in-plane internal vibration modes; a suitable value for torsional force constants has been assigned for calculating the out- of-plane modes. For these calculations, which have been carried out on benzene, naphthalene (h8 and d8), anthracene, phenanthrene and pyrene as examples, the relative importance of intramolecular contribution can be seen in all these compounds, with respect, for instance, to the ring members (here only carbon atoms), or to the peripheral members (here only hydrogen atoms). Because of TDS influence upon the experimental results, the agreement with the observed B's is generally less good than for simpler models, where the internal modes are neglected. For H atoms, the calculated internal contributions to the B's are too high in the case of benzene and naphthalene, and about right for pyrene. The situation is discussed in terms of the effect of packing upon internal modes, and relative non-rigidity of pyrene with respect to smaller molecules.
