[R] Mixed Models providing a correlation structure.

[Marcio]

Hi folks, 
I was wondering how to run a mixed models approach to analyze a linear
regression with a user-defined covariance structure.

I have my model
y = xa +zb +e and 
b ~ N (0, C*sigma_square). (and a is a fixed effects)

I would like to provide R the C (variance-covariance) matrix 

I can easily provide an example, but at this point I am first trying to know
what is the best package the allows an unstructured covariance matrix.

I was trying the function lme in the package nlme but I didn't have success
in the defining the option "correlation"

Thanks


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