[R-sig-ME] Removing p.d. constraint for random effects in lme

[R User]

Hi,

I am trying to fit a mixed model using lme, with a multivariate response.
I would like to try and replicate a SAS proc mixed model that has a type=un
structure for random effects.  I am not very experienced using lme, but it
seems like one of the differences is that lme constrains the random effect
matrix to be positive definite, whereas SAS does not impose this constraint
(only variances in SAS are constrained to be nonnegative).  Is there a way
to remove this positive definite constraint for random effects from lme and
how would this be specified in the model?  My current model looks something
like this:

lme(value ~ trait -1, data,
    random = ~ trait -1| line,
    correlation =  corSymm( form = ~ 1|line/rep),
    weights = varIdent(form = ~ 1 |trait),
    control=control,
    method="REML")

Thanks,
Jacqueline

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