  We study the efficiency of grain alignment by radiative torques (RATs) for an
ensemble of irregular grains. The grains are modeled as ensembles of oblate and
prolate spheroids, deformed as Gaussian random ellipsoids, and their scattering
interactions are solved using numerically exact methods. We define the fraction
of the grains that both rotate fast and demonstrate perfect alignment with
grain long axes perpendicular to the magnetic field. We demonstrate that for
typical interstellar conditions the degree of alignment arising from the RAT
mechanism is significantly larger than that arising from the Davis-Greenstein
(DG) mechanism based on paramagnetic relaxation. We quantify a factor related
to the efficacy of alignment and show that it is related to a
$q_\mathrm{max}$-factor of analytical model (AMO) of the RAT theory. Our
results indicate that the RAT alignment can potentially be sufficiently strong
and to explain observations even if grains do not have magnetic inclusions.
