Knowledge Bank

The information that can be obtained ab initio CAS-SCF computations on Jahn-Teller, Renner-Teller and second order Jahn-Teller systems will be presented in order to stimulate discussion of how this information can be used to rationalize experimental data. Several examples will be presented for each potential surface topology. The $D5h$ Jahn-Teller crossing and associated $C2v$ minima and saddle points were optimised for the cyclopentadienyl radical at the CASSCF/cc-p VDZ level of theory. The $C2v$ structures were characterised by computing analytic force constants. Zero point energies calculated for all $C3H4D$ and $C5H4D$ isomers of the $C2v$ minima suggest an alternative interpretation of the experimentally observed degeneracy resolution. For the benzene radical cation, the Jahn-Teller topology has also been mapped out. Finally, the use of symmetry in characterising the second order John-Teller effect will also be discussed for $D2h$ pentalene and $D8h$ cyclooctatetraene.