Shouldn't matrices of type "Symm" only take one triangle of data? Otherwise you can do this: run runs fine, but sees asymmetrical values in the matrix
s <- mxMatrix(type="Symm", 3, 3,
c(FALSE, TRUE, TRUE, TRUE, FALSE, TRUE, TRUE, TRUE, FALSE),
c(1, .9, .8, .8, 1, .8, .8, .8, 1),
c(NA, "free1", "free2", "free1", NA, "free3", "free2", "free3", NA))
s