title: Covariance (or correlation) matrix of coefficients in lme
body: <p>I fitted a linear mixed model using lme function of nlme package.
  'Summary' command shows the coefficients, their standard errors and correlation matrix of the coefficients.
  How can I get this correlation matrix (covariance matrix is also fine) in a R data set form?</p>
  
  <p>lm command (for linear model) do provide a covariance matrix in the object 'summary(lm5)$cov.unscaled'. I want to find a counterpart of this in lme function.</p>