code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          return -self._denom(R, z)**-0.5
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at (R,z)
          INPUT:
             R - Cylindrical Galactocentric radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             potential at (R,z)
          HISTORY:
             2016-05-09 - Written - Aladdin