Last Update: 1/21/2021
With nonlinear curve fitting, problems with input data or initial values may cause errors (e.g. a zero division error in the fitting function or its derivatives with respect to parameters). In such cases, when you click the Fit until converged button, the fit may fail and return the message "Iterations Performed = 0. Fit did not converge - reason unknown".
When the problem is input data, excluding one bad point may resolve the issue. If the problem is due to bad initial parameter values, adjusting initial values may also resolve the issue.
Sometimes with a user-defined fitting function, it is also possible that the numeric method can't obtain derivatives. Try providing analytic derivatives for the fitting function by checking the Derivatives check box in the Fitting Function Organizer dialog or the Use Derivatives check box in the Fitting Function Builder dialog, and updating the Function Body with derivatives.
Keywords:curve fit, reason unknown, zero iteration, derivatives, zero division