The CASTEP TS Search task allows you to optimize a transition state.

When a 3D periodic molecular or crystal structure is built, it usually needs to be refined to bring it to a stable geometry. The refinement process is known as optimization. It is an iterative procedure in which the coordinates of the atoms are adjusted so that the energy of the structure is brought to a stationary point, i.e., one in which the forces on the atoms are zero. A transition state is a stationary point that is an energy maximum in one direction (the direction of the reaction coordinate) and an energy minimum in all other directions.

During the course of chemical reaction, the total energy naturally changes. Starting from the reactants, the energy increases to a maximum and then decreases to the energy of the products. The maximum energy along the reaction pathway is known as the activation energy. The structure corresponding to this energy is called the transition state.

The TS Search task is particularly useful for predicting barriers to chemical reactions and determining reaction pathways. It can be also used to find diffusion barriers for either solid state diffusion or surface diffusion. The reactants and products can either be optimized separately before running a TS Search or as part of this task. In the latter case partition functions for reactant, product, and transition state can also be calculated. This information can be used to calculate the reaction rate coefficient.

CASTEP transition state searches are performed using synchronous transit methods. These work best when reasonable structures for the reactants and products exist, but the location of the transition state is unknown.

Starting from reactants and products, the synchronous transit methods interpolate a reaction pathway to find a transition state. The Linear Synchronous Transit (LST) method performs a single interpolation to a maximum energy. The Quadratic Synchronous Transit (QST) method alternates searches for an energy maximum with constrained minimizations in order to refine the transition state to a high degree. The options available through CASTEP include:

- LST Maximum performs a single LST maximization, bracketing the maximum between the reactants and product. This is the quickest but least accurate of the options. The TS structure determined by this method generally requires further refinement.
- LST/Optimization performs an LST maximization, followed by an energy minimization in directions conjugate to the reaction pathway. This yields a structure lower in energy and closer to the true TS than a simple LST. Minimization steps continue until an energy minimum is reached or the number of conjugate directions is exhausted.
- Halgren-Lipscomb is a kind of limited LST/Optimization, and is designed to reproduce the algorithm popularized by Halgren and Lipscomb. After determining the LST maximum, this method performs a conjugate gradient minimization, but only in a single direction.
- Complete LST/QST begins by performing an LST/Optimization calculation. The TS approximation obtained in that way is used to perform a QST maximization. From that point, another conjugate gradient minimization is performed. The cycle is repeated until a stationary point is located or the number of allowed QST steps is exhausted.

A complete description of the algorithms used is given in Govind et al. (2003).

Setting up a transition state calculation

CASTEP Transition State Search dialog

Accelrys Materials Studio 8.0 Help: Wednesday, December 17, 2014