def _Rforce(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _Rforce
        PURPOSE:
           evaluate the radial force for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the radial force
        HISTORY:
           2015-12-04 - Started - Bovy (UofT)
        """
        r2= R**2.+z**2.
        r= nu.sqrt(r2)
        return -(1./r-self._a/r2*nu.arctan(r/self._a))/self._a*R/r
NAME:
           _Rforce
        PURPOSE:
           evaluate the radial force for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the radial force
        HISTORY:
           2015-12-04 - Started - Bovy (UofT)