def _phiforce(self, R,z,phi=0,t=0):
        """
        NAME:
           _phiforce
        PURPOSE:
           evaluate the azimuth force at (R,z, phi)
        INPUT:
           R - Cylindrical Galactocentric radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           azimuth force at (R,z, phi)
        HISTORY:
           2016-06-06 - Written - Aladdin 
        """
        if not self.isNonAxi and phi is None:
            phi= 0.
        r, theta, phi = bovy_coords.cyl_to_spher(R,z,phi)
        #x = phi
        dr_dphi = 0; dtheta_dphi = 0; dphi_dphi = 1
        return self._computeforceArray(dr_dphi, dtheta_dphi, dphi_dphi, R,z,phi)