Unlike NLSF Fitter and Multi-X
NLSF Fitter, this tool uses alternative implementation of the Levenberg-Marquardt
algorithm, created by Manolis Lourakis. In
particular, this implementation supports various methods of constrained
optimization, including so-called box constraints. Both DMFitter
ActiveX control and levmar fitting engine library (lmhelper.dll) must be installed.

This fitter works almost in the same way as NLSF Fitter.
However, it is possible to apply box constraints and the set of options is
slightly different (see the screenshots above). Constraints may be defined for
all parameters at once, so if you need to limit only one parameter, set other
values high or low enough, such as the default +/- 1e308. Options have similar
meaning as for standard fitter: thresholds determine whether fitter should stop
when changes in parameters or Chi^{2} are small, derivative delta is used
for numerical computation of the finite difference approximated Jacobians, also
you can define maximal number of LM iterations and a column with relative point
weights. In addition, you can change format of numeric values and initial value
of the damping factor.

Again, keep in mind that in general, nonlinear fitting procedure is not much stable.
You may need some efforts in order to obtain a good fit, that's why you have a
choice of two independent LM implementations and a set of fitting options.