Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Next: Example - Bulk Silicon Up: The .param file Previous: Variable Cell Calculations<br />
Electronic Minimisation Parameters<br />
The calculation may fail to converge if the electronic minimisation parameters are not suitable.<br />
First, the exchange correlation functional. The default choice is the Local Density Approximation and<br />
this will probably be suitable in most cases. If the calculation fails to converge the parameter<br />
xc_functional : value<br />
can be used to try a Generalised Gradient functional. The options are PW91 (Perdew Wang '91), PBE<br />
(Perdew Burke Ernzerhof) and RPBE (Revised Perdew Burke Ernzerhof). When satisfied that a<br />
calculation has completely converged <strong>with</strong> the LDA it is probably a good idea to rerun the whole<br />
calcualtion using a GGA functional to check that the final result is not drastically different. In general the<br />
LDA is better suited to bulk calculations and the GGA to surface calculations.<br />
There is also the option of using a non-local xc_functional (for fixed cell calculations only). The<br />
options are<br />
HF Hartree-Fock<br />
SHF Screened Hartree-Fock<br />
EXX Exact exchange<br />
LDA-X Local Density Approximation for exchange<br />
LDA-C Local Density Approximation for correlation<br />
Health warning; non-local xc functionals are very computationally expensive.<br />
A cap can be placed on the maximum number of allowed SCF cycles. The calculation is stopped if the<br />
electronic minimisation is unconverged after this many step. The keyword string is<br />
max_SCF_cycles : value<br />
If a calculation has taken more than 100 SCF cycles then it probably won't converge, although it does