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An integrated approach employing ab initio, semiempirical (AMl), and force field (CVFF) methods to study torsional barriers in conjugated aromatic molecular systems is presented. It is the first time that such an attempt includingfull geometry optimization up to the ab initio level is reported. We have focused on monomer-like units of poly@-hydroxybenzoic acid) (PHBA) and poly(ethy1ene terephthalate) (PET). Coupling between the torsional motions was studied with the semiempirical AM1 method as well as with the consistent valence force field. Molecular dynamics simulations were carried out on single chains; the relation between MD results and chain flexibility is discussed as well as the consequences of the uncertainty in barrier heights for the MD results and the calculated persistencelength. The numericalresultsarecompared with experimental data as far as such data are available. Large differences between various theoretical results are observed, indicating that there are problems with the modeling of rotational barriers in aromatic moieties containing substituents which are n-conjugated to the aromatic ring. This as well as the lack of sufficient and accurate experimental data hampers progress in modeling the properties of the conjugated aromatic molecules investigated here.

F orce fields with good transferrability for poly (p-hydroxybenzoic acid) and for poly (6-hydroxy-2-naphthoic acid) were developed on the basis of a fit of the force field parameters to the results from semiempirical AMI calculations. The dihedral terms in the force field describing the rotations in the ester linkages are the most important ingredients with respect to modeling of the conformational properties of these macromolecules. These torsional barriers were scaled by a factor of 1.9 as proposed in the second paper of this series. Very good agreement between the scaled AMI potential energy surface and the fitted force field was obtained. When employing such a proper force field, molecular dynamics simulations do show substantial differences in flexibility with respect to bond rotation in single isolated chains as compared to the use of a "standard" force field.

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