A knowledge of the geometries at which excited molecules return to the electronic ground state (So) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S, (lowest excited singlet) and TI (lowest triplet) surfaces, as well as So-Sl conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the So and S, surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two-electron two-orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the So and S, surfaces touch. The results obtained for the simple model are supported by a b initio large-scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and n-donor-to-n-acceptor single bonds, and by similar calculations for "push-pull" perturbed cyclobutadienes, some of which are predicted to have nearly degenerate So, S,, and TI states. The likely consequences of these results for the detailed description of the mechanisms of cisstruns isomerization, the formation of twisted internal charge-transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.

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The electronic and geometric structure of small neutral Li"(n =2 -9) and cationic Li"+ ( n =2 -9) clusters is investigated with the optimal self-consistent-field energy and geometry search and with the multireference diexcited configuration-interaction method. The optimal geometries of neutral and cationic Li clusters are very different. Very small Li"(n &6) clusters prefer planar geometries [deformed sections of the (111) fcc-lattice plane]. The optimal geometries of somewhat larger Li"(6& n & 9) clusters are composed from condensed deformed tetrahedra. This switch in the geometrical structure of clusters can be easily understood on the basis of some simple qualitative arguments. Predicted relatively large stabilities of Li4 and Li8 as well as of Li3+ and Li9+ can help to interpret the "magic numbers" observed when alkali-metal clusters are prepared and detected under differing experimental conditions. The theory predicts stability of relatively small doubly charged alkali-metal clusters which seems to be in variance with the customary ideas of "Coulomb explosion. " The general trend of the dependence of ionization potentials of alkali-metal clusters on the cluster nuclearity agrees with experiments. The present work confirms some conclusions of the "electron-shell model" but puts them on a more general basis, independent of conceptual and methodological details. Furthermore, it reveals new aspects and leads to predictions.

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