Geometry Optimisation with CASTEP

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