The CASTEP module allows you to perform first-principles quantum mechanical calculations in order to explore the properties of crystals and surfaces in the solid state. CASTEP can currently perform several different tasks:

- Single-point energy calculation
- Geometry optimization
- Molecular dynamics
- Elastic constants calculation
- Transition state search
- Transition state confirmation
- Properties

Each of these calculations can be set up so that it generates specified chemical and physical properties. An additional task, known as a properties calculation, allows you to restart a completed job to compute additional properties that were not calculated as part of the original run.

There are a number of steps involved in running a CASTEP calculation, which can be grouped as follows:

- Structure definition: A periodic 3D Atomistic document containing the system of interest must be
specified. There are a number of ways to prepare a structure:
- Periodic structures can be built using the tools available in the Materials Visualizer for building crystals, surfaces, and single-wall nanotubes.
- Existing structures can be modified using the Materials Visualizer sketching tools
- Structures can be imported from an existing structure file

In the case of a transition-state search, a 3D Atomistic Trajectory document containing a reaction sequence is required as the input document. You should define the structures of the reactants and the products in two separate 3D Atomistic documents via the methods listed above and then use the Reaction Preview tool to generate the trajectory.

CASTEP can only be used to perform calculations on 3D periodic structures. If you wish to study a molecular system using CASTEP, you will need to construct a supercell to use as the input structure.

CASTEP performance can be greatly increased if the symmetry of the structure is taken into account. CASTEP offers an option of finding and using the symmetry automatically if the input structure is described as having a P1 symmetry. This option is particularly relevant for calculations on molecular crystals and crystal surfaces, since these structures are typically created as P1 (or p1) systems. Automatic conversion to a higher symmetry structure will honor all atomic properties that are relevant to the current CASTEP settings, such as constraints and electronic configuration.

The time required for a CASTEP calculation increases with the cube of the number of atoms in the system. Therefore, it is recommended that you use the smallest primitive cell description of your system whenever possible. Select Build | Symmetry | Primitive Cell from the menu bar to convert to a primitive cell.

- Calculation setup: Once a suitable 3D structure document has been defined, then it is necessary to select the type of calculation to be performed and set the associated parameters. For example, in the case of a dynamics calculation, these parameters include the ensemble and its settings, the temperature, and the number and length of the time steps. Finally, the server on which the calculation is to be run should be selected and the job initiated.
- Analysis of the results: When the calculation is complete, the files related to that job are returned to the client and, where appropriate, displayed in the Project Explorer. Some further processing of these files may be required to obtain observables such as optical properties. The tools on the CASTEP Analysis dialog may be used to visualize the results of the calculation.

To select a CASTEP task

- Choose Modules | CASTEP | Calculation from the menu bar to display the CASTEP Calculation dialog.
- Select the Setup tab.
- Select the required CASTEP task from the Task dropdown list.

The following topics provide more detail about the various steps involved in setting up and running a CASTEP calculation:

CASTEP tutorials are available to lead you through the various procedures in a step-by-step manner.

CASTEP

CASTEP jobs

Theory in CASTEP

Dialogs in CASTEP

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