[R-sig-ME] random effects covariance matrix specification in nlme

[Chris Stride]

Hi

I'm trying to fit a mixed effects exponential decay model, in which I 
have random effects for the initial value (init), the asymptote (asymp), 
and the rate (rate).

The catch is that I'd also like to estimate the correlation between init 
and asymp, but not between init and rate, or asymp and rate.

Now using  random = pdDiag(init + asymp + rate ~ 1) has none of the 
random effects correlated

And using  random = pdSymm(init + asymp + rate ~ 1) has all three of the 
random effects correlated

How do I specify just the correlation I want?

cheers

Chris