$\alpha$, $\Delta\alpha$ and $\Delta r$ should explain the $N-$ dependence of $I_N$ not only in this range but for all values of $N > N_1$.

Identifying the ranges, $0- N_1$, $N_1-N_2$, $N_2-N_3$, [*etc.*]{} (Fig. 1) as cycles of inccrease and decrease in $I_N$, it appears that M:$^4$He$_N$ cluster has different shells of $^4He$ atoms around M and each cycle represents the completion of one shell. While $^4$He atoms in the first shell have direct bond with M, those in second, third, ... shells are separated from M, respectively, by 1, 2, ... $^4$He atoms in between. The maximum number of $^4He$ atoms, in a particular shell increases in proportion of $R^2$ (where $R$ is the radius of the shell which changes from one shell to next shell in units of the diameter of the sphere which represents the shape and size of $^4$He atom; however it also depends on the shape and size of M. Assuming that M has a shape and size of a $^4$He atom, a rough estimate reveals that the number of $^4$He atoms in first, second and third shells, should be [*around*]{} 6, 18 and 40, respectively. However, it may be emphasized that these numbers agree 