Curve fitting problem and strange message

Question

Fei 2013-5-23

Hello everyone,

I tried to fit my peak with Lorentzian however the background can't be fitted well:

the background of the fitted curve is always lower than the real data.

I used a bound range [0 inf] for this background coefficient and TolFun=1e-25 and TolX=1e-25 for the fitoptions.

Even the fitting is obviously poor as above and matlab still finished with this message:

Success, but fitting stopped because change in residuals less than tolerance (TolFun).

Really don't know how to optimize it. Thanks for any suggestions.

BTW, does anyone know how to post an image here? I wish to attach my fitting curve here.

Image Analyst 2013-5-24

Did you use the Curve Fitting Toolbox for this? If so, list it in the products list below.

Fei 2013-5-24

No, I created routines to fit the data.

Answer 1

Craig Cowled 2013-5-24

Fei, if the "change in residuals is less than tolerance" then this is an indication that you have a reasonably good fit.

I have recently discovered the LSQNONLIN function which is a very quick way to find an optimal solution. Have a look at the code I posted in http://www.mathworks.com.au/matlabcentral/answers/60638-curve-fitting-a-complex-function-using-cftool. It might give you an alternative approach to the problem.

Craig.