17 FIG. 9: The overdensity inside two typical halos selected from the simulations for each model (see text for a more detailed description), as a function of the halo radius R . The solid curves and error bars denote respectively the mean and standard deviation for the results from our 5 sample runs, while the dashed curve is the ΛCDM prediction. In each panel the upper two curves (black) at large radii are for halo I and the lower two curves (green) at large radii are for halo II. ρ B is the background matter (baryons and CDM) density. We skipped the result for R < 10 h − 1 kpc, which is not reliable due to the limit from resolution. profiles also increase as the fifth force becomes less sup- pressed, which is as expected. Our results already show that new features can be quantitatively studied with the N -body method and the improving supercomputing techniques, and that the chameleon model has rather different consequences from other coupled scalar field models. The enhancement in the structure formation due to the fifth force is signifi- cant for some of our parameter space, which means other observables, such as weak lensing, could place new con- straints on the model. Also one might be interested in how the halo profiles would be like at different epoch of the cosmological evolution and in different environments. These will be left as future work. that when chameleon effect is strong and fifth force is suppressed, our result is very close to the ΛCDM predic- tion; this is the case for very large halos (which generally reside in higher density regions) and µ = 10 − $^{6}$. For large halos and µ = 10 − $^{5}$, the situation is more complicated because the competition between two effects of the cou- pled scalar field, namely the speedup of the particles and the deepening of the total attractive potential, has ar- rived a critical point: if the former wins, such as in the model γ = 1 . 0 , µ = 10 − $^{5}$, then the inner density profile can be lower than that in ΛCDM; if the latter wins, such as in the model γ = 1 . 0 , µ = 10 − $^{5}$, then we expect the opposite. For smaller halos which locate in lower den- sity regions, the fifth force is not suppressed that much and causes faster growth of the structure, so the halo is more concentrated and has a higher internal density. Meanwhile, the bias between baryons and CDM density