nlme optim control option

Hello,

I am using the nlme package for quite a while, but recently I have 
discovered the "optim" option in the lmeControl function and I have read 
that until the R verison 2.2.0 the optimization function is "nlminb" and 
before it was "optim" by default. As I am a "user" of R I am not into 
the optimization functions. However, I have a case now where with optim 
= "nlminb" the model does not converge, whereas with optim = "optim" the 
model converges.

Can somebody explain to me in easy words what the difference is ? If 
nlminb does not converge, but optim does, is the result less trustable ?

Thanks,

Stephan

*Stephan Moratti, PhD/
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