  Dispersion corrected density functional theory ($\omega$B97X-D DFT) method is
used to study the molecular hydrogen adsorption in $Ni_nMg_m$ $(1\geq n\geq
3,1\geq m\geq9)$ clusters. All these clusters can effectively adsorb multiple
$H_2$ in the preferred binding energy (BE) range between physisorption and
chemisorption, i.e., $0.1 eV\geq BE \geq0.8 $eV. $H_2$ adsorption on $Ni_kMg_k$
(Ni:Mg=1:1), $Ni_kMg_{2k}$ (Ni:Mg=1:2) and $Ni_kMg_{3k}$ (Ni:Mg=1:3) (k=1-3)
clusters shows fascinating behaviours in terms of Ni:Mg alloying ratio and
cluster size. In each Ni:Mg ratio, the number of adsorbed $H_2$ in the heavier
clusters (k=2, 3) becomes integral multiples of that in the lightest
configuration (k=1). As a consequence, the gravimetric density of molecular
hydrogen remains fixed at each Ni:Mg ratio irrespective of the cluster size.
The corresponding values are $17.94$ $wt\%$ $(1:1)$, $14.46$ $wt\%$ $(1:2)$ and
$13.28$ $wt\%$ $(1:3)$, which are significantly higher than the ultimate target
of $6.5$ $wt\%$ set by DOE, US. Molecular dynamics simulations further reveal
that room temperature desorption of almost all $H_2$ molecules are possible for
all the clusters.
