Synchronous Transit

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. 

HyperChem uses the synchronous transit method described in Peng, C., and Schlegel, H.B., IsraelJoumal of Chemistry, 33, 449-454 (1993). 

Molecular dynamics simulations calculate future positions and velocities of atoms, based on their current positions and velocities. A simulation first determines the force on each atom (Fj) as a function of time, equal to the negative gradient of the potential energy (equation 21). 

You can then determine the acceleration, aj, of each atom by dividing the force acting on it by the mass of the atom (equation 22). 

The change in velocities, Vj, is equal to the integral of acceleration over time. The change in the position, Tj, is equal to the integral of velocity over time. Kinetic energy (K) is defined in terms of the velocities of the atoms (equation 23). 

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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... 

IlyperChem supplies two differeiii types or algorithms for transition state search eigenvector I ollowing and synchronous transit (linear and quadratic search ). 

HyperChein has two synch ron ons transit meth ods im piemen ted. 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 (QSTlisan improvement of LST approach and searches for a maximum along a parabola connecting reactants and products, and for a minimum in all directions perpendicular to the parabola. 

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

Quadratic synchronous transit followed by quasi-Newton. 

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 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. 

It uses a linear or quadratic synchronous transit approach to get closer to the quadratic region of the transition state and then uses a quasi-Newton or eigenvalue-following algorithm to complete the optimization. 

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... 

The Synchronous Transit-Guided Quasi-Newton Method(s)... 

The Linear Synchronous Transit (LST) method forms the geometry difference vector between the reactant and product, and locates the highest energy structure along this line. The assumption is that all variables change at the same rate along tire reaction path. 

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