I’m trying to translate a non-linear mixed-effects model from nlme to brms syntax. I think I got it except for the part that specifies random effects covariance.

Here’s the nlme version:

```
nlme(y~c1+a*exp(-(x^2/(2*(exp(logsigma)^2)))),
data = curdata,
fixed = a + c1 + logsigma ~ 1,
random = pdDiag(a + c1 + logsigma ~ 1),
groups = ~ participant)
```

And here’s my take on brms syntax:

```
brm_hg_free_sigma_nlme<-brm(bf(y~c1+a*exp(-(x^2/(2*(exp(logsigma)^2)))),
c1~1+(1|participant),
logsigma~1+(1|participant),
a~1+(1|participant),
nl = T),
prior = c(prior(student_t(2, 0, 0.5), coef = "Intercept", class = "b", nlpar = "a"),
prior(student_t(4, 0, 3), coef = "Intercept", class = "b", nlpar = "c1")),
data = curdata, cores = 4, save_all_pars = T, sample_prior = T, iter = 2000, warmup = 1000
)
```

I see that there’s a cov_ranef parameter but I don’t understand what kind of matrix it is expecting. In vignette(“brms_phylogenetics”), the covariance matrix seems to be computed based on the data, if I understand correctly, while I simply want to set a diagonal one. Is there an easy way to do it?