def _zforce(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _zforce
        PURPOSE:
           evaluate the vertical force for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the vertical force
        HISTORY:
           2015-02-07 - Written - Bovy (IAS)
        """
        return self._mn3[0].zforce(R,z,phi=phi,t=t)\
            +self._mn3[1].zforce(R,z,phi=phi,t=t)\
            +self._mn3[2].zforce(R,z,phi=phi,t=t)