Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
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depend on the nature of the system and the method used to treat any metallic species in the system<br />
metals_method : value<br />
For insulators no special treatment is required. Including a parameter<br />
fix_occupancy : true<br />
defines the system as an insulator and metals method will be set to a default value of NONE.<br />
If the user defines<br />
fix_occupancy : false<br />
then by default a choice will be made by <strong>CASTEP</strong> for<br />
metals_method : EDFT<br />
Now <strong>with</strong> Ensemble Density Functional Theory (EDFT) the calculation is more likely to converge than<br />
<strong>with</strong> the alternative method Density Mixing (DM) and will usually require fewer SCF cycles to minimise<br />
the electronic structure for a given ionic structure. However, DM is much faster per SCF cycle than<br />
EDFT. For systems containing metallic atoms it is advisable to first try DM <strong>with</strong><br />
max_scf_cycles : 50<br />
and if this is failing to converge switch to EDFT.<br />
The electronic minimisation is considered to have converged when the change in the total energy from<br />
one iteration to the next remains below some tolerance value per atom for a few scf steps. The default<br />
value of the parameter<br />
elec_energy_tol : value<br />
is<br />
eV per atom and is usually suitable but it might be suitable to reduce the strictness of this<br />
tolerance limit if a calculation is consistently failing to converge. The number of iterations for which the<br />
change in the total energy must remain below elec_energy_tol is the convergence window and it<br />
can be specified by.<br />
elec_convergence_win
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