MNDOC-CI method

These results also follow from the cross sections through the potential energy surfaces calculated in the semiempirical all-valence electron approximation by the MNDOC-CI method for acetaldehyde, acrolein, and benzal-dehyde, which are displayed in Figure 7.13. Stabilization of the state... 

Based on the work of Thiel, such a semiempirical MR-CI was developed some time ago. This MNDOC-CI method may be characterized as follows. For each state under consideration, one or a few reference configurations are used, and from these reference configurations single and double excitations within a properly chosen active space (AS) are considered in constructing spin-adapted configuration state functions (CSFs). To specify the active space and the refer-... 

Starting with y= 120° and 0 = 0°, the MNDOC-CI method yields a conical intersection, the structure of which is shown in Figure 6.8. The values 0=0° for the rotational angle and 7 = 143,7° for the bond angle are as expected the bond distances are between those of azirine and nitrile ylide, especially because the distance between nitrogen and the methylene carbon r N = 1.34 A has decreased and is equal to the double-bond value r N = 1.34 A in the nitrile ylide. To completely establish the mechanism, it remains to be shown that the conical intersection is accessible without a barrier from the Franck-Condon geometry and that the nitrile ylide can be reached from the conical intersection. This is easily... 

Since the ab initio determination of the structural dependence of these quantities is feasable only for small systems, we present here in Section 2 a formalism for a straightforward determination of SOC within the context of configuration interaction (Cl) calculations [9]. This formalism has been implemented for the semiempirical MNDOC-CI method [10] and allows for routine determinations of PE surfaces and SOC surfaces at the same level of theory. Some results for 1,1-, 1,2-, 1,3- and 1,4-biradicals will be given in Section 3 and compared to ab initio results available from the literature. In Section 4 some simple models will be discussed, which allow for a rationalization of the structural dependence of SOC in the biradicals discussed in the previous sections, and finally, in Section 5 some general conclusions are presented concerning the mechanisms of organic triplet photoreactions. 

Izzo R, Klessinger M. Optimization of conical intersections using the semiempirical MNDOC-CI method with analytic gradients. J Comput Chem. 2000 21 52-62. 

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