Setting the quality of a calculation
An important concept that affects a large number of settings exposed in CASTEP is that of a calculation quality. The Quality setting on the Setup tab of the CASTEP Calculation dialog provides a shortcut for setting up a balance between the accuracy and speed of calculations. To help you select appropriate values for CASTEP parameters, four predefined parameter sets are available. The predefined sets vary in quality from Coarse, suitable for quick approximate calculations, to Ultra-fine, for calculations where high precision is required.
CASTEP formalism lends itself naturally to a systematic convergence study by means of increasing the size of the basis set, accuracy of Brillouin zone sampling, and so on. Nevertheless there are cases for which there is no need to systematically change the quality settings and a fast qualitative description is sufficient. The Express quality setting is provided to allow you to obtain reasonable results quickly in many simple cases. This setting can be an order of magnitude faster than the Fine setting, while producing results that are sufficiently accurate for exploratory studies, for investigation of qualitative trends in a high-throughput context for example. The Express setting is especially useful for semiconductors and insulators and can be fairly accurate for nonmagnetic metals. The study of magnetic metallic systems usually requires more accurate calculation settings than those defined by the Express quality setting.
The Express setting cannot be translated into the Coarse - Ultra-fine accuracy scale.
To help you select appropriate values for CASTEP parameters, four predefined parameter sets are available. The predefined sets vary in quality from Coarse, suitable for quick approximate calculations, to Ultra-fine, for calculations where high precision is required.
To select a quality
- Choose Modules | CASTEP | Calculation from the menu bar to display the CASTEP Calculation dialog.
- Select the Setup tab.
- Choose a Quality setting from the dropdown list to select one of the four predefined parameter sets.
When you choose a Quality setting, the parameters are set according to the task and the system.
| Quality | Express | Coarse | Medium | Fine | Ultra-Fine |
|---|---|---|---|---|---|
| Energy tolerances per | Cell | Atom | Atom | Atom | Atom |
| SCF tolerance (eV/atom) | 1.0 × 10-5 | 2.0 × 10-6 | 1.0 × 10-6 | 5.0 × 10-7 | |
| SCF tolerance (eV/cell) | 0.0001 | ||||
| SCF Convergence window | 2 | 3 | 3 | 3 | 3 |
| k-points separation (metals) (Å-1) | 0.07 | 0.07 | 0.05 | 0.04 | 0.04 |
| k-points separation (insulators) (Å-1) | 0.1 | 0.1 | 0.08 | 0.07 | 0.07 |
| Energy cutoff | Fine | Coarse | Medium | Fine | 1.1 × Fine |
| FFT Grid Density | Standard | Standard | Standard | Fine | Precise |
| Apply finite basis set correction | 1 | 1 | 1 | 1.2 | 1.5 |
| Augmentation density scaling factor | Never | Smart | Smart | Smart | Smart |
The values of the Energy cutoff that correspond to the Coarse, Medium, and Fine settings are element specific and are taken from the pseudopotential files stored in the database. These values were determined from convergence tests for single atoms and diatomic molecules and they roughly correspond to total energy convergences of 2, 0.3, and 0.1 eV atom-1, respectively. The highest value for the selected quality setting among the elements present in the 3D structure document is accepted.
The Quality setting also affects the values of the convergence tolerances for tasks such as geometry optimization, elastic constants calculations, and transition-state searching.
The values of force, stress, and displacement tolerances for the Geometry Optimization and Elastic Constants tasks are set to 100 for ExpressQuality. This preset value instructs CASTEP to ignore these tolerances and consider only the energy tolerance as a convergence criterion.
Geometry optimization
| Quality | Express | Coarse | Medium | Fine | Ultra-fine |
|---|---|---|---|---|---|
| Energy tolerance (eV/atom) | 5.0e-5 | 2.0e-5 | 1.0e-5 | 5.0e-6 | |
| Energy tolerance (eV/cell) | 0.001 | ||||
| Max. force tolerance (eV/Å) | 100 | 0.1 | 0.05 | 0.03 | 0.01 |
| Max. stress tolerance (GPa) | 100 | 0.2 | 0.1 | 0.05 | 0.02 |
| Max. displacement tolerance (Å) | 100 | 5.0e-3 | 2.0e-3 | 1.0e-3 | 5.0e-4 |
| BFGS Use line search | No | Yes | Yes | Yes | Yes |
The values of force, stress, and displacement tolerances are set to 100 for ExpressQuality. This preset value instructs CASTEP to ignore these tolerances and consider only the energy tolerance as a convergence criterion.
The individual tolerances may need to be adjusted in some cases:
- The Max. force tolerance can be reduced for molecular systems (isolated molecules, molecules on surfaces, molecular crystals, and so on)
- The Max. stress tolerance is generally expressed as a fraction of the bulk modulus of the material, so it should be smaller for soft materials. However, in practice, cell optimization of soft substances (for example, molecular crystals) is difficult and might not be stable enough to impose very stringent convergence criteria.
Elastic constants
| Quality | Express | Coarse | Medium | Fine | Ultra-fine |
|---|---|---|---|---|---|
| Energy tolerance (eV/atom) | 1.0e-5 | 4.0e-6 | 2.0e-6 | 1.0e-6 | |
| Energy tolerance (eV/cell) | 0.001 | ||||
| Max. force tolerance (eV/Å) | 100 | 0.02 | 0.01 | 0.006 | 0.002 |
| Max. displacement tolerance (Å) | 100 | 1.0e-3 | 4.0e-4 | 2.0e-4 | 1.0e-4 |
The values of force, and displacement tolerances are set to 100 for ExpressQuality. This preset value instructs CASTEP to ignore these tolerances and consider only the energy tolerance as a convergence criterion.
These tolerances are five times more strict than the ones used for the Geometry Optimization task.
TS Search
| Quality | Express | Coarse | Medium | Fine | Ultra-fine |
|---|---|---|---|---|---|
| RMS convergence (eV/Å) | 0.1 | 0.5 | 0.25 | 0.05 | 0.01 |
See Also:
Setting up electronic options
Setup tab - CASTEP Calculation dialog