#
CASTEP Dynamics task

The CASTEP Dynamics task allows you to simulate how the atoms in a 3D periodic structure will move under the influence of
computed forces.

Before performing a CASTEP dynamics calculation you can select the thermodynamic ensemble and associated parameters and define the simulation time, temperature, and pressure.

## Selecting the thermodynamic ensemble

Integrating Newton's equations of motion allows you to explore the constant-energy surface (NVE dynamics) of a system. However, most natural
phenomena occur under conditions where a system exchanges heat with the environment. These conditions can be simulated using NVT ensembles
(either the deterministic NosÃ© ensemble or the stochastic Langevin ensemble).

External pressure can be incorporated into the dynamics calculation by using either the NPH (constant enthalpy) or the NPT (constant
temperature) ensemble.

## Defining the time step

An important parameter in the integration algorithm is the time step. To make the best use of the computer time, a large time step should be
used. However, if the time step is too large it may lead to instability and inaccuracy in the integration process. Typically, this is manifested
as a systematic drift in the constant of motion.

Quantum-mechanical molecular dynamics calculations usually require a smaller time step than
forcefield based dynamics applications. However, this is less of an issue in CASTEP because wavefunction
and density extrapolation are used.

## Constraints during dynamics

CASTEP supports constraints during molecular dynamics simulations. However, you can only
apply some of the more basic constraints through the Materials Studio interface:

- The center of mass can be fixed.
- Individual atom positions can be fixed.
- Bonds lengths can be fixed.

###### See Also:

Dynamics

Setting up a molecular dynamics calculation

CASTEP Dynamics dialog

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