code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          l,n = bovy_coords.Rz_to_lambdanu(R,z,ac=self._ac,Delta=self._Delta)
          return -1./(nu.sqrt(l) + nu.sqrt(n))
docs: NAME:
              _evaluate
          PURPOSE:
              evaluate the potential at R,z
          INPUT:
              R - Galactocentric cylindrical radius
              z - vertical height
              phi - azimuth
              t - time
          OUTPUT:
              Phi(R,z)
          HISTORY:
              2015-02-15 - Written - Trick (MPIA)