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:
           2013-08-28 - Written - Bovy (IAs)
        """
        return -self.alpha*R*z*(R**2.+z**2.)**(-1.-self.alpha/2.)
def _Rzderiv ( self , R , z , phi = 0. , t = 0. ) : return - self . alpha * R * z * ( R ** 2. + z ** 2. ) ** ( - 1. - self . alpha / 2. )
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:
           2013-08-28 - Written - Bovy (IAs)