pro read_halo

    filename = "vhalos_036_1e12.dat"
    nhalos = 0l ; number of halos in the file
    halo_str = {pos:fltarr(3),$ ; position of the halo in units of Mpc/h
                mvir:0.0, $     ; mass of the halo in units of Msun/h, defined as the mass 
                                ; within a sphere of average density equal to 200 times the
                                ; critical density of the universe 
                vmax:0.0}       ; maximum circular velocity, in units of kms/sec  

    openr,1,filename
    readu,1,nhalos
    group = replicate({pos:fltarr(3), mvir:0., vmax:0.},nhalos)
    readu,1, group
    close,1


     z = 0. ; change according to the target snapshot number
     Hubble = 100.
     Omega0 = 0.25
     G = 43.0071
     omegaz = Omega0 * (1+z)^3/ (Omega0*(1+z)^3 + 1-Omega0 )
     rho_back = 3 * Omega0 * Hubble^2 / (8 * !PI * G)
     rho_crit = rho_back/omegaz
     DeltaCrit200 = 200.0 / omegaz
     
     R200 = ( group.mvir / DeltaCrit200/ rho_back / (4.*!Pi/3)  )^(1./3)

     V200 = sqrt(G * group.mvir / R200) 
stop

end