docs: def macs(nasv,T):
      ''' 
      Returns
      -------
      MACS
          [mb] at T [K] from Na*<sigma v>.
                                          
      '''
  
      Na = avogadro_constant
      k  = boltzmann_constant
      vtherm=(2.*k*T/mass_H_atom)**0.5
  
      s      = old_div(nasv,(vtherm*Na))
      macs   = s*1.e27
      return macs