code: def rtide(self,R,z,phi=0.,t=0.,M=None):
          
          if M is None:
              #Make sure an object mass is given
              raise PotentialError("Mass parameter M= needs to be set to compute tidal radius")
          r= nu.sqrt(R**2.+z**2.)
          omegac2= -self.rforce(R,z,phi=phi,t=t,use_physical=False)/r
          d2phidr2= self.r2deriv(R,z,phi=phi,t=t,use_physical=False)
          return (M/(omegac2-d2phidr2))**(1./3.)
docs: NAME:
              
              rtide
              
          PURPOSE:
              
              Calculate the tidal radius for object of mass M assuming a circular orbit as
  
              .. math::
  
                 r_t^3 = \\frac{GM_s}{\\Omega^2-\\mathrm{d}^2\\Phi/\\mathrm{d}r^2}
  
              where :math:`M_s` is the cluster mass, :math:`\\Omega` is the circular frequency, and :math:`\Phi` is the gravitational potential. For non-spherical potentials, we evaluate :math:`\\Omega^2 = (1/r)(\\mathrm{d}\\Phi/\\mathrm{d}r)` and evaluate the derivatives at the given position of the cluster.
  
          INPUT:
          
              R - Galactocentric radius (can be Quantity)
              
              z - height (can be Quantity)
              
              phi - azimuth (optional; can be Quantity)
              
              t - time (optional; can be Quantity)
              
              M - (default = None) Mass of object (can be Quantity)
              
          OUTPUT:
              
              Tidal Radius
          
          HISTORY:
              
              2018-03-21 - Written - Webb (UofT)