Options tab
The Options tab allows you to specify the minimization algorithm and the parameters controlling the algorithm.
Algorithm: Specify the minimization algorithm. These algorithms are supported by CASTEP:
- LBFGS
- BFGS
- Damped MD
- TPSD
The LBFGS algorithm is generally recommended, especially for large systems. It implements a universal sparse preconditioner that accelerates geometry optimization (Packwood et al., 2016). However, if there are constraints TPSD might be a better choice (see the Geometry optimization theory topic for further details).
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).
The Damped MD algorithm is not used when cell optimization is performed.
Use line search: When checked, indicates that optimization should use the advanced line search algorithm.
This checkbox is checked by default, except when the main quality is set to Express. If you specify the Express quality setting and then request line search, the Quality is displayed as Customized on both the Minimizer tab and the Setup tab of the CASTEP Calculation dialog. In the latter scenario, you must set convergence criteria for gradients and displacements to sensible values, as the line search algorithm cannot function using only the energy change criterion.
Use delocalized internals: When checked, indicates that delocalized internal coordinates should be used during geometry optimization, instead of Cartesian coordinates. This setting typically speeds up the calculation by 2-3 times.
The Use delocalized internals option is enabled only if the BFGS algorithm is selected.
The delocalized internals minimizer does not currently support cell optimization. As a
result, CASTEP will automatically switch over to Cartesian coordinates when the cell is allowed to vary.
The delocalized internals minimizer respects nonlinear constraints only for P1 structures that do not contain mixture atoms.
Damping algorithm: Specify the algorithm for the Damped MD method. The supported algorithms are:
- Independent - the damping constant is chosen independently for each mode, so that all modes lose energy at the same rate.
- Coupled - the same damping constant is used for all modes at each step.
- Steepest Descent
The Independent damping algorithm is recommended (see the Geometry optimization theory topic for details).
Damping coefficient recalculation frequency: During the damped MD optimization, the damping coefficients are recalculated occasionally to improve the damping rate. This value specifies the number of MD steps between these recalculations.
The Damping coefficient recalculation frequency value is not used by the steepest descent algorithm.
Wavefunction extrapolation: Specify the method to be used for wavefunction extrapolation between MD steps.
- None - no extrapolation is performed.
- 1st VV - first-order extrapolation with variable coefficients.
- 2nd VV - second-order extrapolation with variable coefficients.
- 2/1 VV - alternate second- and first-order extrapolation with variable coefficients.
- 1st CC - first-order extrapolation with constant coefficients.
- 2nd CC - second-order extrapolation with constant coefficients.
- 2/1 CC - alternate second- and first-order extrapolation with constant coefficients.
Time step: Specify the time step to be used in the damped MD geometry optimization.
Auto-update time step: When checked, indicates that the time step will be automatically updated to speed up the geometry optimization.
The Time step and Auto-update time step controls are enabled only if the Damped MD algorithm is selected.
Access methods
| Menu | Modules | CASTEP | Calculation | Setup | More... | Options |
| Toolbar |
 | Calculation | Setup | More... | Options
|
See Also:
Geometry optimization theory
Geometry optimization task
CASTEP Geometry Optimization dialog
Minimizer tab - CASTEP Geometry Optimization dialog
Stress tab - CASTEP Geometry Optimization dialog