I’m trying to construct a phylogenetic model.

When I calculate the expected variances and covariances, the output included unrooted branch length.

`tree_vcvc <- ape::vcv(tree.c$scenario.1, corr = TRUE)`

```
tree.c$scenario.1
Phylogenetic tree with 1010 tips and 542 internal nodes.
Tip labels:
Adenophora_palustris, Adenophora_takedae_var._howozana, Adenophora_maximowicziana, Adenophora_uryuensis, Adenophora_nikoensis_var._teramotoi, Adenophora_tashiroi, ...
Node labels:
, 0.95, 0.95, 0.95, 0.22, 0.95, ...
Unrooted; includes branch lengths.
```

Then I try to model using treevcvc, but it showed the error “Error: Within-group covariance matrices must be positive definite.”

```
bmodel<- brm(example ~ Temperature + Precipitation+
(1 + Temperature + Precipitation |gr(sn, cov = tree_vcvc)),
data = d,
family = cumulative(link = "logit", threshold = "flexible"),
data2 = list(tree_vcvc = tree_vcvc),
prior = c(set_prior("normal(0,10)", class = "b")),
warmup = 300,
iter = 1000,
chains = 4,
cores = 4,
save_all_pars = TRUE,
backend = "cmdstanr",
control = list(adapt_delta = 0.99, max_treedepth=15),
save_model = "all_stanscript",
file = "all")
```

`> Error: Within-group covariance matrices must be positive definite.`

When I gave the different phylogenetic data, it did include rooted branch length, and the model calculation worked well. Is there a way to avoid non-positive definite in unrooted phylogenetic data?