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General Job Parameters

An important first choice is which method to use for optimisation of the ionic positions. The default option is the Broyden-Fletcher-Goldfarb-Shannon (BFGS) method and this is used throughout the rest of this user guide. For fixed cell calculations there are two other options, damped molecular dynamics

geom_method : dampedmd

and delocalised internal coordinates

geom_method : delocalized

The user is free to learn about these methods from the literature and try them out if they suit a particular problem. Since we will be doing variable cell calculations later on we will stick with BFGS.

The verbosity of information in the output files is controlled by

iprint : value

value can be 0,1,2 or 3 in order of increasing verbosity with a default value of 1, which in most cases will be sufficient to understand how the calculation has progressed. iprint : 3 is for full debugging. For users who wish to know a little bit more about what occurs during a calculation it might be interesting to set

iprint : 2.

If a calculation is unexpectedly interrupted many hours of work might be lost. CASTEP automatically backs up the calculation after every 5 BFGS steps. The parameter

num_backup_iter : value

allows the backup interval to be user specified. For very slow jobs it is sensible to back up the results after every BFGS step

num_backup_iter : 1

When a calculation that was stopped is resumed the parameter

continuation : default

must be included so that the calculation will read the .check file and use the last completed BFGS step as a starting point for the calculation. This feature is essential when the calculation is run on large shared computational facilities where the run time per job is restricted.

It is possible to choose whether the algorithm should favour a speedier calculation at the cost of using more RAM during the calculation or whether to to slow down the calculation and make it less RAM intensive.

opt_strategy : value

value can be 'MEMORY' or 'SPEED'. The default is for a balance between memory usage and performance. If the calculation is at all likely to be RAM limited then MEMORY should be chosen. On RAM extensive architectures SPEED should be chosen. SPEED should also be used for most clusters, multiple processors or supercomputers because these write data temporarily to disk so as to conserve RAM which significantly hampers performance.

next up previous
Next: Basis Set Parameters Up: The .param file Previous: The .param file