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

The UNO-CAS method, a computationally efficient approximation to CAS-S CF, has been applied to calculate completely optimized geometries for a variety of conjugated pi-systems. The study includes all-trans-polye nes 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 shor tening of the single bonds(approximately 0.005 angstrom). The strong d ependence of the geometry on electron correlation shows the importance of the latter for potential surface studies. Except for one case, gly oxal, the correlation is largely nondynamical rather than dynamical an d can be well reproduced by UNO-CAS. Changes in the nature of conjugat ion in polyenes are close to convergence in dodecahexaene. The experim ental geometries of some benzene derivatives and naphthalene are discu ssed.