Transition structures and energy barriers of pericyclic reactions in the CASSCF approach

Five energy hypersurfaces of the most examined pericyclic reactions have been investigated by using ab initio SCF, CASSCF, and the semiempirical AM1 methods. The systems are H4, H6, C3H6, C4H4, and C3OH4. Stationary points and their sets of harmonic vibrational frequencies have been calculated by means of analytical gradient techniques in the frameworks of the respective approaches. ZPE corrected energy barriers are based on single‐point calculations including dynamical correlation corrections by CAS‐CI(SD)+DC, CASCEPA, or MP2.