We use ab initio CASSCF and CASPT2 computations to construct the composite multistaterelaxation path relevant to cycloocta-1,3,5,7-tetraene singlet photochemistry. The results show that anefficient population of the dark excited state (S
1) takes place after ultrafast decay from the spectroscopicexcited state (S
2). A planar
D8h-symmetric minimum represents the collecting point on S
1. Nonadiabatictransitions to S
0 appear to be controlled by two different tetraradical-type conical intersections, which aredirectly accessible from the S
1 minimum following specific excited-state reaction paths. The higher-energyconical intersection belongs to the same type of intersections previously documented in linear and cyclicconjugated hydrocarbons and features a triangular -(CH)
3- kink. This point mediates both cis
![](/images/entities/rarr.gif)
transphotoisomerization and cyclopropanation reactions. The lowest energy conical intersection has a boat-shaped structure. This intersection accounts for production of semibullvalene or for double-bond shifting.The mapping of both photochemical and thermal reaction paths (including also Cope rearrangements,valence isomerizations, ring inversions, and double-bond shifting) has allowed us to draw a comprehensivereactivity scheme for cyclooctatetraene, which rationalizes the experimental observations and documentsthe complex network of photochemical and thermal reaction path interconnections. The factors controllingthe selection and accessibility of a number of conjugated hydrocarbon prototype conical intersections andground-state relaxation channels are discussed.