A simple density functional fractional occupation number procedure to determine the low energy transition region of spin-flip reactions

A computationally simple three-step procedure to survey the energy landscape and to determine the molecular transition structure and activation energy at the intersection of two weakly coupled electronic potential energy surfaces of different symmetry is suggested. Only commercial software is needed to obtain the transition states of, for instance, spin-flip reactions. The computational expense is only two to three times larger than that of the standard determination of an adiabatic reaction path. First, the structures of the two electronic initial and final states along a chosen reaction coordinate are individually optimized. At the "projected crossing," the two states have the same energy at the same value of the reaction coordinate, but different state-optimized partial structures. Second, the unique optimized structure of a low energy crossing point between the two states is determined with the help of the density functional fractional occupation number approach. Finally, the respective energy of the two states at the crossing is estimated by a single point calculation. The prescription is successfully applied to some simple topical examples from organic and from inorganic chemistry, respectively, concerning the spin-flip reactions (3)H(3)CS(+)-->(1)H(2)CSH(+) and (7)MoCO(2)-->(5)MoCO(2)-->(3)OMoCO.