again: nonlinear fit...

*To*: mathgroup at smc.vnet.net*Subject*: [mg52100] again: nonlinear fit...*From*: db at ict.fhg.de (julia)*Date*: Thu, 11 Nov 2004 04:51:58 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

Hello, I've posted some days before and i didn't get an answer. I think my question was not detailed enough and therefor I'll try it again. My experimental data have to be fit to this model: lamkonv[x_, {v_, Dz_, L_}] := 1/(2*v^2*x^3)*((Dz*x/Pi)^0.5*(L*Exp[-L^2/(4*Dz*x)] - (L + v*x) *Exp[-(L - 2*v*x)^2/(4*Dz*x)]) + 0.5*(L^2 + Dz*x)* (Erf[L/(2*(Dz*x)^0.5)] - Erf[(L - 2*v*x)/(2*(Dz*x)^0.5)])) where Dz,v,L are the parameters. (It is a 2-dim. convective-diffusion model) I've tried an own code, nonlinearFit, NonlinearRegress and other optimization algorithms (quasiNewton..) ad I couldn't work it out. 1.The fit runs in a physically wrong range or 2.I get imaginary values for the parameters or 3. I get the "nrlnum" -message or 4. The fit does nothing Especially the parameter Dz behaves very strange. The value of Dz i about 10^3 times smaller than the other parameters. I've read in a posting before, that this may cause problems. Can anybody help me with that?