Hi!
I have just started to discover the metric_file
option from cmdstan and I must say it is really awesome. A few questions:

Should the
engaged
option be set to 0 if I give Stan the inverse mass matrix and the stepsize? I thought this would be the case, but I got very odd behavior such that I am confused. So when doing this Stan runs supersuper fast, but gives me crap. Maybe this is a bug. 
Is there some documentation on the mapping from the parameters to the unconstrained parameter space. In particular I would be interested in ordering.

Do we have recommendations or some guidance (likely with big warnings) what can be the minimal warmup when giving the metric file?

Can cmdstan export the inverse masses? Currently I use rstan to parse things out.

Can rstan already use the inverse mass matrix (@bgoodri? )?
This is getting really interesting in the context of MPI. Now you can use all available CPUs you have to get a single chain through warmup ASAP. Then you just start of as many chains as you like using the already calculated inverse weights. Certainly this is dangerous, but likely very handy for modeldevelopment.
Great new feature! Thanks to @mitzimorris, @bbbales2 and however else made this happen!
Best,
Sebastian