[R] nlme-problem

[nioksane@cc.jyu.fi]

Hi,
I have a problem with fitting a nonlinear mixed-effects model to my data.
I was able to fit quite simple exponential model but now I’m trying to fit
the following Gompertz model:

> gomp<-nlme(paino~b0+b1*exp(-exp(-b2*(daydeg.scale-b3))), data=group1,
fixed=b0+b1+b2+b3~1, start=c(b0=150, b1=3000, b2=0.6, b3=2500))

I’m getting the following error:

Error in chol((value + t(value))/2) : the leading minor of order 2 is not
positive definite

Could you give me some ideas what might be wrong and how to deal with it?
I know that the data and the syntax should be ok as I have tested it
already with exponential model. The problem seems to be in the model-form.

Many thanks,

Niina Kotamaki
University of Jyvaskyla