code: def _z2deriv(self,R,z,phi=0.,t=0.):
          
          a = self._a
          return self._denom(R, z)**-1.5 - 3. * (a + nu.fabs(z))**2. * self._denom(R, z)**-2.5
docs: NAME:
             _z2deriv
          PURPOSE:
             evaluate the second vertical derivative for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the second vertical derivative
          HISTORY:
             2016-05-13 - Written - Aladdin