Parameter types

There are 9 types of GDS parameetrs, that is, 9 possible values for the gds attribute. Several of these will be familiar to users of IFEFFIT and ARTEMIS, a few are newly introduced by DEMETER.

guess

A parameter which is varied in a fit.

def

A parameter whose math expression is continuously updated throughout the fit.

set

A parameter which is evaluated at the beginning of the fit and remains unchanged after that.

lguess

A locally guessed parameter. In a multiple data set fit, this will be expanded to one guess parameter per data set. See the section on local guess parameters for more details.

restrain

A restrain parameter is defined in an IFEFFIT script as a def parameter but is used as a restraint in the call to IFEFFIT's feffit command. In a multiple data set fit, all restraints are defined in the first call to the feffit command.

skip

A skip is a parameter that is defined but then ignored. Setting a variable to a skip is useful in a GUI as a way of “commenting out” a parameter without removing it from the fitting project.

after

This is like a def parameter, but is not used in the fitting model and only evaluated when the fit finishes. It is then reported in the log file.

penalty

This is like a def parameter, but is used as a user-defined penalty to the happiness parameter, which is evaluated at the end of the fit. This is not currently implemented.

merge

A merge is the type given to a parameter that cannot be unambiguously resolved when two Fit objects are merged into a single Fit object. A fit cannot proceed until all merge parameters are resolved. It is unlikely that a variable would ever be declared as a merge by a user, although if a user script makes use of any of DEMETER's (as yet unimplemented) project merging features, it will certainly be necessary to resolve a merge parameter by renaming it globally and resetting the gds attribute.

Penalty and merge types are currently unimplemented.