Molecular hydrogen formation efficiency as a function of grain size for (top) α = 0.4 and (bottom) α = 1.0. Monte Carlo results are indicated with circles (10 K), squares (12 K), diamonds (14 K) and crosses (15 K), solid and dashed lines represent the classical and modified rate equation results, respectively. In the bottom panel, the 10, 12, and 14 K (modified) rate equations results overlap.