Setting up a geometry optimization
Geometry optimization is one of the tasks most often performed with CASTEP.
Convergence criteria
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.
Geometry optimization algorithm
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.
The TPSD geometry optimization algorithm is a viable alternative for systems where the potential energy surface deviates significantly from the quadratic form. TPSD offers robust convergence and is particularly recommended when user-supplied constraints on lattice parameters are applied during cell optimization (for example, when optimizing a solid-solid interface in a direction perpendicular to the interface plane).
Constraints
Geometry optimization in CASTEP obeys constraints on fractional atomic positions imposed using 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 .cell file.
The default setting in CASTEP is to optimize atomic coordinates only. You must choose an option from the Cell optimization dropdown list on the Minimizer tab of the Geometry Optimization dialog if cell optimization is required.
Non-linear constraints
Geometry optimization using delocalized internals in CASTEP obeys constraints on interatomic distances, bonds, and/or torsions imposed using the Materials Visualizer. You can set such constraints using the Edit Constraints dialog, accessible from the Modify menu.
- All non-linear constraints are only taken into account if you choose the delocalized internal minimizer, by checking the Use delocalized internals checkbox on the Options tab of the CASTEP Geometry Optimization dialog.
- The Damped MD algorithm for geometry optimization acknowledges only constrained bonds (distances) between atoms, all other constraints (torsions and angles) are ignored.
- Non-linear constraints cannot be applied in conjunction with symmetry constraints. Thus, if your crystal has symmetry other than P1, you will be prompted to convert the crystal symmetry to P1 before you can continue.
- Non-linear constraints cannot currently be applied to disordered systems.
Potential pitfalls
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:
- change electronic minimization algorithm to All Bands/EDFT scheme
- use a stricter SCF tolerance
- use a less strict Max. stress tolerance
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.
See Also:
Geometry optimization theory
Setup tab - CASTEP Calculation dialog
CASTEP Geometry Optimization dialog