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Abstract
A new computerized method for locating conical intersections of interest in photochem. is presented. The search is based on the Longuet-Higgins phase change theorem (Berry phase) which provides the subspace required for the initial search. The subspace is approximated as a plane contg. three stable structures lying on a Longuet-Higgins loop. The search is conducted for a min. of DE, the energy ...
Abstract
A new computerized method for locating conical intersections of interest in photochem. is presented. The search is based on the Longuet-Higgins phase change theorem (Berry phase) which provides the subspace required for the initial search. The subspace is approximated as a plane contg. three stable structures lying on a Longuet-Higgins loop. The search is conducted for a min. of DE, the energy difference between two electronic states. It is started using up to three points within the circle defined by the three structures; symmetry, if relevant, is helpful but not essential. Since a two-dimensional subspace of the large 3N - 6 space is used, the search that uses either Cartesian or internal coordinates is efficient and yields a degeneracy after a few iterations. Given that not all degrees of freedom are included in the search, usually a high lying part of the conical intersection is initially located. The system is subsequently optimized along all coordinates keeping DE as close to zero as desired. The method is demonstrated for the sym. H3 system and also for the butadiene-cyclobutene-bicyclobutane system in which the three stable structures are not equiv. The method is general and can be extended to any photochem. system.
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