Transition structure optimization techniques

Transition structures can be dehned by nuclear symmetry. For example, a symmetric Spj2 reaction will have a transition structure that has a higher symmetry than that of the reactants or products. Furthermore, the transition structure is the lowest-energy structure that obeys the constraints of higher symmetry. Thus, the transition structure can be calculated by forcing the molecule to have a particular symmetry and using a geometry optimization technique. 

For our initial geometry for the transition structure, we ll detach one hydrogen from the carbon and increase the O-C-H bond angle. We specified the Opt=(TS,CalcFC) keyword in the route section, requesting an optimization to a transition state. The CalcFC option is used to compute the initial force constants, a technique which is generally helpful for transition state optimizations. We ve also included the Freq keyword so that a frequency calculation will automatically be run at the optimized geometry. 

Chapter 3, Geometry Optimizations, describes how to locate equilibrium structures of molecules, or, more technically, stationary points on the potential energy surface. It includes an overview of the various commonly used optimization techniques and a consideration of optimizing transition strucmres as well as minimizations. 

The reaction modeling techniques described so far (transition structure optimization, adiabatic mapping, and reaction path modeling) rely on the assumption that a single protein structure... 

There are actually very few. Modern optimization techniques practically guarantee location of a minimum energy structure, and only where the initial geometry provided is too symmetric will this not be the outcome. With a few notable exceptions (Hartree-Fock models applied to molecules with transition metals), Hartree-Fock, density functional and MP2 models provide a remarkably good account of equilibrium structure. Semi-empirical quantum chemical models and molecular mechanics models, generally fare well where they have been explicitly parameterized. Only outside the bounds of their parameterization is extra caution warranted. Be on the alert for surprises. While the majority of molecules assume the structures expected of them, some will not. Treat "unexpected" results with skepticism, but be willing to alter preconceived beliefs. 

Jensen F. Transition structure optimization techniques. In Schleyer PVR, ed. Encyclopedia of Computational Chemistry. New York John Wiley, 1998 3114-3123. 

Recently the concerted approach has been studied at the SCF level using the STO-3G basis set and gradient techniques with optimization of all variables by Brown and Houk. In this case an authentic transition structure has been obtained, possessing one imaginary vibrational frequency. Ortega et... 

Geometry Optimization 2 Gradient Theory Nonadiaba-tic Derivative Couplings Reaction Path Following Transition State Theory Transition Structure Optimization Techniques. 

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