Optimization of conical intersections using the semiempirical MNDOC-CI method with analytic gradients

An efficient procedure is presented for calculating analytic gradients in t he framework of a semiempirical multireference configuration interaction me thod. The same algorithm is used to locate the lowest energy point of a con ical intersection by the direct method of Bearpark et al. (Chem Phys Lett 1 994, 223, 269). Results for the conical intersections on the photoreactions of ethylene and butadiene are in very good agreement with nb initio data. In addition, conical intersections were optimized of the photoreactions of azirine as well as for the cyclooctatetraen-semibullvalene and the dewamaph thalene-naphthalene photorearrangement reactions. (C) 2000 John Wiley & Son s, Inc.