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I am fitting a mixed effect model with a random slope on a factor with 6 levels using the function lmer().

lmer(cc_marg ~ mPair + (0 + mPair | ratID), data = dat, REML = FALSE, 
     na.action = na.omit, control = lCtr)

I would like to simplify the variance-covariance matrix of the random effect by specifying that the 6 levels of the factor mPair are uncorrelated. In other words, I would like to specify a diagonal var-cov matrix. How can I do that using lmer()?

I know it would be easier to use lme() from the nlme package, as I would have access to pdClass. However, that package has issues at estimating the denominator DF for random slope models, and therefore, I would prefer to use the lme4 package. Thanks!

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