"pdCompSymm" function in R package "nlme"

Here are the available matrix classes available in nlme:

library(nlme)
apropos("^pd[^f]")
  [1] "pdBlocked"   "pdCompSymm"  "pdConstruct" "pdDiag"      "pdIdent"
  [6] "pdLogChol"   "pdMat"       "pdMatrix"    "pdNatural"   "pdSymm"

?pdClasses


  Unfortunately, I don't think there's anything useful in there:

pdIdent is a multiple of the identity matrix, pdDiag is (heterogeneous) 
diagonal.

I believe pdNatural, pdLogChol, pdSymm represent different 
parameterizations for the general positive-definite ("unstructured") 
correlation matrix.

   So "equal correlations but heterogeneous variances" doesn't appear to 
be an option.

   The MCMCglmm package has more options.
   glmmTMB offers a 'heterogeneous compound symmetric" but *not* (at 
present) a *homogeneous compound symmetric" option (although that could 
be achieved without too much difficulty by using the 'map' parameter to 
set some parameters equal ...)

Dear all,

I am a graduate student. I have a question about pdCompSymm.
pdCompSymm can be used for compound symmetry matrix only (All the variances
are equal and all the covariances are equal.)

Are there any functions for unequal variances or unequal covariances matrix?

Thank you in advance.

Isaac

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Dr. Benjamin Bolker
Professor, Mathematics & Statistics and Biology, McMaster University
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