B3LYP calculations molecular associations

It seems as if this problem can be solved by considering mechanisms that occur before the electron has time to fully relax. The idea being that transient anions associated with the lowest unoccupied molecular orbitals will excite vibrational modes. These ideas are outlined in a new paper by Kumar and Sevilla that looks at C5-05 bond dissociation in 5 -dTMP. They report that on the vertical potential energy surface, the B3LYP/6-31G(d) calculated barrier height for C5 -05 bond dissociation is ca. 9 kcal/mole which is lower than the adiabatic value for this same process [112],... 

Based on this energy diagram and the molecular parameters obtained at the B3LYP/6-311+G(3df, 2p) level, variational TST and RRKM calculations have been carried out for the unimolecular decomposition and the reverse bimolecular association reaction rate constant of the HO-CIO3 system. 

Combined quantum mechanical/molecular mechanical methods are not, of course, restricted to studies of reactions but can also be used to study association processes and conformational transitions. Most implementations use a two-zone model as described above, but Morokuma and colleagues have described a multilayered approach called ONIOM [Svensson et al. 1996]. ONIOM is a particularly apt name given that a typical calculation is constructed from a series of layers For example, a three-layer ONIOM calculation on the Diels-Alder reaction involved an inner core treated with the B3LYP density functional approach, the intermediate layer with a Hartree-Fock level of theory and the outer layer with MM3. A particular feature of ONIOM and its related methods is that they provide rigorous gradients and second derivatives, so enabling properties such as vibrational frequencies to be calculated [Dapprich et al. 1999]... 

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