software to fit optical spectra - Quantum Materials Group

4.10.2. Fitting options
There is a possibility to set up the technical parameters, which regulate the flow of the
fitting process. To do this one should click the button in the “Fit” window (Figure
4-19). The dialog window “Fitting options” will show up (Figure 4-20).
Figure 4-20 The “Fitting options” window.
To use these options requires the knowledge of the Levenberg-Marquardt algorithm,
described in section 2.1.1. However, in most cases the default settings work reasonably well.
The first set of parameters (“Stop fitting after”) define the criteria to terminate the fitting
process. The following criteria can be applied:
(i) a certain maximum number of iterations is exceeded;
(ii) the fitting takes too much time;
(iii) the process seems to converge, i.e., the total chi-square did not vary significantly
during the last few iterations;
(iv) one of the parameters goes out of range 20 ;
(v) the linearity parameter λ becomes too large.
The group “Confidence limits” deals with the calculation of the parameter confidence
limits, or error bars (see section 2.1.3). One can force RefFIT to calculate them after each
fitting iteration. This may slightly slow down the fitting process. The value in the “DeltaChiSq”
2
corresponds to δχ in Equation 2-14.
By default, all spectra and parameters are updated on the screen after each iteration. In
order to save some time it is possible to let the program to update the screen not so often by
changing the field “Update screen every … iteration(s)” 21 .
20 This option is not realized yet
Guide to RefFIT Page 78