I was not able to find anything online regarding this.
Although there is documentation on Stan MPI https://github.com/stan-dev/math/wiki/Threading-Support
I am not sure whether RStan 2.18 is ready for MPI parallelisation, and if so whether there is some documentation around.
There is no specific RStan documentation for MPI. It is basically this
but referring to ~/.R/Makevars rather than make/local .
a short mention of this in the document, I’m sure will be helpful for other RStan users. I can contribute with this small edit myself if helpful.