# Minimizer tab

The Minimizer tab allows you to specify the convergence level of the minimization and the external pressure, and to control cell optimization.

Quality: Set the geometry optimization convergence thresholds for energy change, maximum force, maximum stress, and maximum displacement between optimization cycles. The optimization will stop when all of these criteria are satisfied.

Four sets of convergence thresholds are available:

• Coarse
• Medium
• Fine
• Ultra-fine

The values of the convergence thresholds in each set are given in the table below:

Value Coarse Medium Fine Ultra-fine
Energy (eV/atom) 5 × 10-5 2 × 10-5 1 × 10-5 5 × 10-6
Max. force (eV/Å) 0.1 0.05 0.03 0.01
Max. stress (GPa) 0.2 0.1 0.05 0.02
Max. displacement (Å) 0.005 0.002 0.001 5 × 10-4

Alternatively, thresholds can be specified independently for Energy, Max. force, Max. stress, and Max. displacement. If you enter your own values for any of these settings, the Quality is displayed as Customized on both the Minimizer tab and the Setup tab of the CASTEP Calculation dialog.

When the main quality is set to Express the geometry optimization quality is also set to Express. In this case energy tolerance will be displayed in eV/cell rather than eV/atom, the default value is 0.001 eV/cell. All other tolerances are ignored and geometry optimization runs stop when the energy change is less than the tolerance regardless of the values of forces or displacements.

Energy: Specify the convergence threshold for the maximum energy change, in eV/atom, during the geometry optimization. The units are eV/cell if the Quality is set to Express.

Max. force: Specify the convergence threshold for the maximum force, in eV/Å, during the geometry optimization. If the Quality is set to Express, this tolerance by default will be set to a very high value and will effectively be ignored in the calculation.

Max. stress: Specify the convergence threshold for the maximum stress, in GPa, during the geometry optimization. If the Quality is set to Express, this tolerance by default will be set to a very high value and will effectively be ignored in the calculation.

Max. displacement: Specify the convergence threshold for the maximum displacement, in Å, during the geometry optimization. If the Quality is set to Express, this tolerance by default will be set to a very high value and will effectively be ignored in the calculation.

Max. iterations: Specify the maximum number of geometry optimization cycles. If this number of cycles is reached, then the calculation will stop, even if the convergence criteria are not satisfied.

Optimize cell: When checked, indicates that the cell parameters will be optimized during the geometry optimization, in addition to the atomic coordinates. Default = unchecked.

Basis set for variable cell: Specify the desired behavior of the basis set when the cell size changes. Default = Fixed Basis Quality.

• Fixed Basis Quality - The number of plane waves changes to maintain the fixed cutoff energy (finite basis set correction is required)
• Fixed Basis Size - The number of plane waves is kept constant, so the effective cutoff energy changes (finite basis set correction is not required)

Fixed Basis Quality is the recommended option.

Compressibility: Specify an estimated compressibility value for the crystal under investigation in order to accelerate the geometry optimization. Each entry in the dropdown list is interpreted as a preset value:

• Soft - 25 GPa, suitable for soft materials like molecular crystals
• Medium - 200 GPa, suitable for most transition metals
• Hard (default) - 500 GPa, suitable for minerals, ceramics, and hard metals

CASTEP geometry optimization will respect constraints on the fractional coordinates of atoms. These constraints may be imposed using the controls on the Atom tab of the Edit Constraints dialog.

Nonlinear constraints are respected only for P1 structures that do not contain mixture atoms and only when the delocalized internals optimizer is used, i.e., the Use delocalized internals checkbox is checked on the Options tab of the CASTEP Geometry Optimization dialog.

Access methods

 Menu Modules | CASTEP | Calculation | Setup | More... | Minimizer Toolbar | Calculation | Setup | More... | Minimizer