def _R2deriv(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _R2deriv
        PURPOSE:
           evaluate the second radial derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the second radial derivative
        HISTORY:
           2015-06-15 - Written - Bovy (IAS)
        """
        return (self._b2-2.*R**2.+z**2.)*(R**2.+z**2.+self._b2)**-2.5
def _R2deriv ( self , R , z , phi = 0. , t = 0. ) : return ( self . _b2 - 2. * R ** 2. + z ** 2. ) * ( R ** 2. + z ** 2. + self . _b2 ) ** - 2.5
NAME:
           _R2deriv
        PURPOSE:
           evaluate the second radial derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the second radial derivative
        HISTORY:
           2015-06-15 - Written - Bovy (IAS)