Thank you Ian Hinder.

Yes I want to add a low density fluid to a BBH simulation.

I review the source code of GRHydro_InitialData Thorn's GRHydro_OnlyAtmo.F90 .  I found it give any position ( I mean every nx,ny,nz) a rho value which is a constant equal to "rho_abs_min" or "initial_rho_abs_min".

so I think it can give a low density fluid to a BBH simulation.

But it report "NAN" or "Con2Prim: count > GRHydro_countmax".