You need to input rough guesses for the fit parameters. Sometimes just guessing "1" for
each parameter will work.
For fitting functions with a "c" parameter, you can choose to fix the value.
This option allows you to use "c" as a parameter without varying the value
during least squares adjustment.
If the calculation doesn't converge,
Try using convergence damping.
In cases of slow convergence, enter the results from the previous
non-converged run as guesses for the next run.
Use the "Fit values from initial guesses" output to refine your guesses. In other
words, vary your input guesses to decrease the initial residuals.