Target average acceptance probability
The Stan control argument adapt_delta sets the target average acceptance probability for the No-U-Turn Sampler (NUTS) used by Stan.
The default value of adapt_delta used by nma() is 0.8 for fixed effect models, and 0.95 for random effects models.
You should not need to change adapt_delta unless you see a warning message about divergent transitions. Increasing adapt_delta from the default to a value closer to 1 means that Stan will use a smaller step size, making sampling slower but more robust, and resulting in fewer divergent transitions.
For more details see the Stan documentation available from https://mc-stan.org/users/documentation/.
Useful links