Synchronous transit methods

HyperChem uses the synchronous transit method described in Peng, C., and Schlegel, H.B., IsraelJournal of Chemistry, 33, 449-454 (1993), and the details of the method can be found there. 

The synchronous transit method is combined with quasi-Newton methods to find transition states. Quasi-Newton methods are very robust and efficient in finding energy minima. Based solely on local information, there is no unique way of moving uphill from either reactants or products to reach a specific reaction state, since all directions away from a minimum go uphill. 

HyperChem has two synchronous transit methods implemented. The linear synchronous transit method (LST) searches for a maximum along a linear path between reactants and products. It may happen that this method will end up with a structure having two or more negative eigenvalues. The quadratic synchronous transit method (QST) is an improvement ofLST approach and searches for a maximum along a parabola connecting reactants and products, and for a minimum in all directions perpendicular to the parabola. 

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Halgren T A and Lipscomb W N 1977 The synchronous transit method for determining reaction pathways and locating molecular transition states Chem. Phys. Lett. 49 225... 

FIGURE 17.3 Illustration of the linear synchronous transit method for generating a starting point for a transition-structure optimization. 

HyperChem offers a Reaction Map facility under the Setup menu. This is needed for the synchronous transit method to match reactants and products, and depending on X (a parameter having values between 0 and 1, determining how far away from reactants structures a transition structure can be expected) will connect atoms in reactants and products and give an estimated or expected transition structure. This procedure can also be used if the eigenvector following method is later chosen for a transition state search method, i.e., if you just want to get an estimate of the transition state geometry. 

In HyperChem, two different methods for the location of transition structures are available. Both arethecombinationsofseparate algorithms for the maximum energy search and quasi-Newton methods. The first method is the eigenvector-following method, and the second is the synchronous transit method. 

The Synchronous-Transit Method for determining Reaction Pathways and Locating Molecular Transition States Thomas A. Halgren and William N. Lipscomb Chemical Physics Letters 49 (1977) 225-232... 

Figure 14.5 Illustration of the linear and quadratic synchronous transit methods. Energy maxima and minima are denoted by and , respectively...

A simple but efficient and useful alternative is represented by the synchronous transit method (725), where the saddle point region is reached... 

Halgren, T. A., and W. N. Lipscomb (1977), The Synchronous-Transit Method for Determining Reaction Pathways and Locating Molecular Transition States, Chem. 

Twenty years of research in this interdisciplinary field have not solved the problem associated with PES and at this time there is no efficient and reliable approach. Usually the procedure for finding the TS structure is a combination of different methods arranged in two stages. In the first step an approximate location of a saddle point is searched, while the second one is devoted to its refinement. The initial stage can be considered as the main part of the TS search. In some cases it is possible to determine the vicinity of a TS point using simple approaches such as "reaction coordinate" and synchronous transit methods [6,11] or even making successful guesses. As a rule such approaches work well in the case of relatively simple shapes of PESs and in clear chemical situations. 

The LST and QST calculations do not actually locate a proper transition state but aim to arrive at structures sufficiently close to it. Ideally, the resulting configuration would lie within the quadratic basin of the first order saddle point and be suitable for input to subsequent transition state searches. However, the synchronous transit methods often yield structures with more than one negative eigenvalue. 

Halasz GJ, Vibok A, Mayer I (1999) Comparison of basis set superposition error corrected perturbation theories for calculating intermolecular interaction energies. J Comput Chem 20 274-283 Halgren TA, Lipscomb WN (1977) The synchronous-transit method for determining reaction pathways and locating molecular transition states. (3hem Phys Lett 49 225-232 Hammond GS (1955) A correlation of reaction rates. J Am Chem Soc 77 334-338... 

In addition to the above-mentioned methods, there exist some other schemes for locating a transition state based on original ideas, such as the X-method [45], the Halgren-Lipscomb or synchronous transit method [46], the method of a hypersphere (or a circle in the two-dimensional case) [47] and others. These are considered in detail in the review articles [35-47]. 

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