Applicability of MNDO techniques AM1 and PM3 to ring-structured polymers

Semiempirical Hartree-Fock techniques are widely used to study properties of long ring-structured chains, although these types of systems were not included in the original parametrization ensembles. These techniques are very useful for an ample class of studies, and their predictive power should be tested. We present here a study of the applicability of some techniques from the NDDO family (MNDO, AM1, and PM3) to the calculation of the ground state geometries of a specific set of molecules with the ring-structure characteristic. For this we have chosen to compare results against ab initio Restricted Hartree-Fock 6-31G(d,p) calculations, extended to Møller-Plesset 2 perturbation theory for special cases. The systems investigated comprise the orthobenzoquinone (O(2)C(6)H(4)) molecule and dimers (O(2)C(6)H(4))(2), as well as trimers of polyaniline, which present characteristics that extend to several systems of interest in the field of conducting polymers, such as ring structure and heterosubstitution. We focus on the torsion between rings, because this angle is known to affect strongly the electronic and optical properties of conjugated polymers. We find that AM1 is always in qualitative agreement with the ab initio results, and is thus indicated for further studies of longer, more complicated chains.