def _Rzderiv(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _Rzderiv
        PURPOSE:
           evaluate the mixed R,z derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           d2phi/dR/dz
        HISTORY:
           2015-02-07 - Written - Bovy (IAS)
        """
        return self._mn3[0].Rzderiv(R,z,phi=phi,t=t)\
            +self._mn3[1].Rzderiv(R,z,phi=phi,t=t)\
            +self._mn3[2].Rzderiv(R,z,phi=phi,t=t)
NAME:
           _Rzderiv
        PURPOSE:
           evaluate the mixed R,z derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           d2phi/dR/dz
        HISTORY:
           2015-02-07 - Written - Bovy (IAS)