Alex has earlier stated that there are no plans to incorporate range-separated functionals in Firefly, so that would mean that the PW86PBE-XDM functional will not appear in a future version of Firefly.
As for your second question, this is indeed a tough one. Functionals such as SSB-D and S12g/h are currently not implemented. Double hybrid functionals or DFT-NL can give good results but analytical gradients are unfortunately not yet available for these. As such, it is difficult to name a good 'runner-up' to PW86PBE-XDM.
Edit: it seems I misinterpreted the PW86PBE-XDM functional in my original post as it is not a range-separated hybrid one.
On Tue Oct 4 '16 0:32am, TG3D wrote
>The subject functional appears to provide excellent energy and distance behavior for all three major non-covalent interactions (electrostatic, H-bonding and dispersive (vdW)). Are there any plans to incorporate this functional into a future version of Firefly? If not, I have a really difficult question: which of the built-in functional and options would be a runner-up to PW86PBE-XDM?
[ This message was edited on Sun Oct 9 '16 at 5:59pm by the author ]