FindFit / NonlinearFit Problems

*To*: mathgroup at smc.vnet.net*Subject*: [mg65671] FindFit / NonlinearFit Problems*From*: "Xoedusk" <xoedusk at gmail.com>*Date*: Wed, 12 Apr 2006 06:00:41 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Hello. I am trying to do a NonlinearFit or Findfit, but I get the following cryptic error: FindFit::sszero: The step size in the search has become less than the \ tolerance prescribed by the PrecisionGoal option, but the gradient is larger \ than the tolerance specified by the AccuracyGoal option. There is a \ possibility that the method has stalled at a point which is not a local \ minimum. My data is {{0.345721, 0.0351973}, {0.523381, 0.0267363}, {0.699319, 0.0149018}, {0.871647, 0.0036456}, {1.04792, 0.00167384}, {1.22115, 0.0413637}, {1.39689, 0.234294}} My model is \!\(A*\((\((\(n\^2\) Cos[Î¸]\ - \ \@\(n\^2 - Sin[Î¸]\^2\))\)\/\((\(n\^2\) \ Cos[Î¸]\ + \ \@\(n\^2 - Sin[Î¸]\^2\))\))\)\^2 + C\) Or in more-readable terms, A* ((n^2 Cos[Î¸] - Sqrt[n^2 - Sin[Î¸]^2])/(n^2 Cos[Î¸] + Sqrt[n^2 - Sin[Î¸]^2]))^2 + C My pars are {n, A, C} My vars are Î¸ I am typing \!\(\(\(\ \)\(FindFit[dataRUnoErrorRad, A*\((\((\(n\^2\) Cos[Î¸]\ - \ \ \@\(n\^2 - Sin[Î¸]\^2\))\)\/\((\(n\^2\) Cos[Î¸]\ + \ \@\(n\^2 - Sin[Î¸]\^2\))\))\)\^2 \ + C, {n, A, C}, Î¸\ ]\)\)\) Any help would be very very appreciated! I am able to get a nice fit by hand, but i really need good numbers via mathematica.