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The scaled quantum mechanical force field (SQM) of a series of polycyclic aromatic hydrocarbons (PAH) of astrophysical interest was calculated at the MNDO level. The set of scale factors for each type of coordinate involved were determined for the smallest molecules of the series, benzene and naphthalene and their deuterated analogues. It the largest molecules, the transfer of the internal coordinates produces a redundant set. The scaling process was modified in order to be able to use a non-redundant set, not destroying symmetry. The resulting mean error in the frequencies is reduced from 30 to 25 cm⁻¹, if the set of scale factors extracted from benzene is replaced by that calculated from naphthalene.