EFFECT OF NONDYNAMIC ELECTRON CORRELATION ON THE GEOMETRIES OF CONJUGATED PI-SYSTEMS

The UNO-CAS method, a computationally efficient approximation to CAS-SCF, has been applied to calculate completely optimized geometries for a variety of conjugated pi-systems. The study includes all-trans-polyenes up to C12, molecules with heteroconjugation, with triple bonds and aromatics. Relative to SCF values, the bond lengths of multiple bonds are increased by 0.02-0.03 angstrom, with a simultaneous smaller shortening of the single bonds(approximately 0.005 angstrom). The strong dependence of the geometry on electron correlation shows the importance of the latter for potential surface studies. Except for one case, glyoxal, the correlation is largely nondynamical rather than dynamical and can be well reproduced by UNO-CAS. Changes in the nature of conjugation in polyenes are close to convergence in dodecahexaene. The experimental geometries of some benzene derivatives and naphthalene are discussed.