Greetings,

I am trying to implement Joint Species Models where the variance covariance of regression coefficients is informed by a correlation matrix derived from species phylogenies. One parameter that needs to be estimated is

real<lower=0,upper=1> rho;

which measures the strength of the phylogenetic signal. Ovaskainen et al. in their software HMSC https://www.helsinki.fi/en/researchgroups/statistical-ecology/hmsc use for this parameter the following prior:

with probability 0.5, rho = 0,

with probability 0.5, rho ~ Uniform(0,1).

the idea behind this is to allow rho to be zero for the case when phylogeny does not influence parameter estimates.

Is there a way to define a prior in Stan that will behave similarly?

I have tried a beta with lots of mass near zero but it is not quite the same.