G. Fogarasi et al., EFFECT OF NONDYNAMIC ELECTRON CORRELATION ON THE GEOMETRIES OF CONJUGATED PI-SYSTEMS, Journal of physical chemistry, 97(16), 1993, pp. 4036-4043
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.