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From: "Carlos Alejandro Perez Lasso" <carlosalejandro85@gmail.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Optimization
Date: Thu, 22 Nov 2012 16:12:07 +0000 (UTC)
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"Matt J" wrote in message <k8jjv2$ioc$1@newscl01ah.mathworks.com>...
> "Carlos Alejandro Perez Lasso" <carlosalejandro85@gmail.com> wrote in message <k8jaks$fqr$1@newscl01ah.mathworks.com>...
> > Hi Matlab users,
> > 
> > My purpose is to use the Optimtool to estimate material model parameters so that running the model with such parameters in Abaqus results in a predicted curve close to some experimental data. 
> > I am trying to solve the problem with the lsqnonlin solver 
> ============
> 
> If it's a curve fitting problem, mightn't it be better to use lsqcurvefit.
> 
> 
> > Optimization completed because the size of the gradient at the initial point 
> > is less than the default value of the function tolerance.
> ==========
> 
> This usually occurs because you have quantizing operations like ROUND, FLOOR, CEIL, etc... in your objective function. They make the function locally flat in places, making the initial point trivially a point of zero gradient.

Thank you Matt for your answer. 
I will try lsqcurvefit and see what happens. 
Regarding the objective function, it doesn't have quantizing operations like the once you described above. It does contain operations that write python scripts to produce input files for running Abaqus simulation and retrieve information once the simulation is done.
Thanks for your help Matt.