Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Next: Inputs Up: <strong>Geometry</strong> <strong>Optimisation</strong> <strong>with</strong> <strong>CASTEP</strong> Previous: <strong>Geometry</strong> <strong>Optimisation</strong> <strong>with</strong><br />
<strong>CASTEP</strong><br />
Recipe<br />
The essence of the calculation is for the ions and electrons in the supercell to be moved around stepwise<br />
until the forces on the atoms and the change in total energy between steps fall below some predefined<br />
convergence tolerance. The ionic positions are optimised using quasi-Newton methods. For each<br />
configuration of the ionic positions the electronic configuration is optimised using the method of<br />
conjugated gradients. The flow of the calculation is thus;<br />
1. Move ions into new positions using geometry optimisation algorithm<br />
2. Optimise electronic configuration using conjugated gradients method<br />
3. Compare total energy <strong>with</strong> previous configurations and check if forces <strong>with</strong>in tolerance limits<br />
4. If structure not optimised start at (1) and generate new set of ionic positions<br />
This cycle is performed until the forces fall <strong>with</strong>in the tolerance limit and the energy should then be a<br />
local minimum.<br />
What types of system can this be applied to? In theory <strong>CASTEP</strong> can geometry optimise most bulk<br />
systems in a wide range of materials such as semiconductors, ceramics, metals, minerals and zeolites. It<br />
can also look at defects such as surfaces. The main restriction is the amount of computational resources<br />
available to the user. Calculations scale <strong>with</strong> the cube of the number of atoms included. More about this<br />
in the discussion of supercells.<br />
Next: Inputs Up: <strong>Geometry</strong> <strong>Optimisation</strong> <strong>with</strong> <strong>CASTEP</strong> Previous: <strong>Geometry</strong> <strong>Optimisation</strong> <strong>with</strong><br />
<strong>CASTEP</strong><br />
2005-04-04