I’m working on construct a phylogenetic model using brms.

Previously, the below model was converged, but now it became to produce an error, “Within-group covariance matrices must be positive definite”. I feel the part of code `tree_vcvc_new <- tree_vcvc + 0.000001 * tree_vcvc`

is not appropriate. How is the right way to transform the matrix to positive definite?

```
tree_vcvc <- ape::vcv(tree.c$scenario.1, corr = FALSE)
tree_vcvc_new <- tree_vcvc + 0.000001 * tree_vcvc
colnames(tree_vcvc_new)<-colnames(tree_vcvc)
rownames(tree_vcvc_new)<-rownames(tree_vcvc)
#model constructing
bmodel<- brm(example ~ Temperature + Precipitation +
(1 +Temperature + Precipitation |gr(sn, cov = tree_vcvc_new)),
data = d,
family = cumulative(link = "logit", threshold = "flexible"),
data2 = list(tree_vcvc_new = tree_vcvc_new),
prior = c(set_prior("normal(0,10)", class = "b")),
warmup = 300,
iter = 1000,
chains = 4,
cores = 4,
threads = threading(4),
save_all_pars = TRUE,
backend = "cmdstanr",
control = list(adapt_delta = 0.99, max_treedepth=15),
save_model = "all_vcvcn_stanscript",
file = "all_vcvcn")
```

Thanks,