Geometry optimization is one of the tasks most often performed with CASTEP.
The accuracy of the calculation is controlled by the Quality option on the Geometry Optimization dialog.
For a detailed description of the quality settings please refer to the Setting the quality of a calculation topic.
The BFGS algorithm is the recommended algorithm for minimizing structures. The damped molecular dynamics approach is always slower and does not allow cell relaxation. The only case when damped dynamics may be appropriate is in systems with a very flat potential energy surface. BFGS can be extremely slow in such cases and can get stuck in a local minimum.
Geometry optimization in CASTEP obeys constraints on fractional atomic positions imposed via the Materials Visualizer. You can set such constraints using the Edit Constraints dialog, accessible from the Modify menu. Note that atoms must be completely fixed; CASTEP does not support partial constraints on the x, y, and z components of Cartesian atom positions.
At present, you must check the Fix fractional position checkbox on the Atom tab of the Edit Constraints dialog in order to apply fixed atom constraints in CASTEP. This has the effect of fixing both fractional and Cartesian positions in a fixed cell calculation, and fixing fractional positions in a variable cell run. In the current version of Materials Studio, checking the Fix Cartesian position checkbox has no effect on CASTEP calculations.
More general linear constraints in CASTEP can be specified manually by editing the
The default setting in CASTEP is to optimize atomic coordinates only. You must check the Optimize cell option on the Minimizer tab of the Geometry Optimization dialog if cell optimization is required.
Geometry optimization using delocalized internals in CASTEP obeys constraints on interatomic distances, bonds, and/or torsions imposed via the Materials Visualizer. You can set such constraints using the Edit Constraints dialog, accessible from the Modify menu.
One of the most difficult parts of geometry optimization in CASTEP is cell optimization. It is important to ensure that you use a finite basis set correction in such calculations, and preferably a fine quality level.
The accuracy of the stresses which are used in the cell optimization algorithm are crucial for the success of geometry optimization. Certain systems, for example, rare-earth compounds with highly localized electronic states, require very stringent criteria for SCF convergence in order to converge the stress tensor. This is particularly true for the Density mixing minimizer.
If cell optimization fails with the following message in the output file:
"BFGS: Geometry optimization failed to converge after ... steps",
You should try one of the following:
Another potential pitfall is the use of the Fixed Basis Size Cell optimization setting when the starting geometry is very different from the final one. Finite basis set correction does depend on the cell variables, although this dependence is disregarded by the minimizer. In addition, the effective cutoff energy changes when the cell geometry is modified with the above setting (it is the number of plane waves that is kept fixed). If this change takes the dEtot/d(ln Ecut) function far away from the point that was used to evaluate the finite basis set correction, the results obtained will not be accurate. Therefore, you should compare the starting and final geometries and perform a completely new run starting from the final configuration if the difference between the two is large.
Fixed Basis Quality is a recommended setting for cell optimization. Even then it is a good idea to perform an additional geometry optimization calculation after successfully completing the first, particularly if there were noticeable changes in the geometry.
Geometry optimization theory
Setup tab - CASTEP Calculation dialog
CASTEP Geometry Optimization dialog