Searching for conical intersections of potential energy surfaces with the ONIOM method: application to previtamin D.

We demonstrate that the ONIOM method can be used to optimize a conical intersection between the ground and first excited-state potential energy surfaces of previtamin D (precalciferol), with excitation localized in a small part of the molecule: the hexatriene chromophore. These calculations were up to 100 times faster with little loss of accuracy compared to a full non-ONIOM Target calculation. The most accurate ONIOM method combination was CASSCF/4-31G//ROHF/STO-3G(Triplet): in comparison to the Target (CASSCF/4-31G), bond lengths and angles in the hexatriene model region were calculated to within 0.02 A and 0.7 degrees , respectively, and the energy difference between the conical intersection and nearest associated S 1 minimum to within 0.5 kcal x mol (-1). All of the low-level methods selected produced accurate geometries, including the UFF molecular mechanics and AM1 semiempirical methods, suggesting a cheap and efficient way of initially optimizing conical intersections geometries. Furthermore, ONIOM allows for an assessment of the localization of excited states, providing some fundamental insight into the physical processes involved.