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We present the results of theoretical investigation on the electronicstructure, bonding nature and ground state properties of Th$_2$Al andTh$_2$AlH$_4$ using generalized-gradient-corrected first-principlesfull-potential density-functional calculations. Th$_2$AlH$_4$ has been reportedto violate the "2 \AA rule" of H-H separation in hydrides. From our totalenergy as well as force-minimization calculations, we found a shortest H-Hseparation of 1.95 {\AA} in accordance with recent high resolution powderneutron diffraction experiments. When the Th$_2$Al matrix is hydrogenated, thevolume expansion is highly anisotropic, which is quite opposite to otherhydrides having the same crystal structure. The bonding nature of thesematerials are analyzed from the density of states, crystal-orbital Hamiltonianpopulation and valence-charge-density analyses. Our calculation predictsdifferent nature of bonding for the H atoms along $a$ and $c$. The strongestbonding in Th$_2$AlH$_4$ is between Th and H along $c$ which form dumb-bellshaped H-Th-H subunits. Due to this strong covalent interaction there is verysmall amount of electrons present between H atoms along $c$ which makesrepulsive interaction between the H atoms smaller and this is the precisereason why the 2 {\AA} rule is violated. The large difference in theinteratomic distances between the interstitial region where one can accommodateH in the $ac$ and $ab$ planes along with the strong covalent interactionbetween Th and H are the main reasons for highly anisotropic volume expansionon hydrogenation of Th$_2$Al.Comment: 14 pages, 9 figure

physical review. b, condensed matter

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Electronic structure, phase stability, and chemical bonding inTh2AlandTh<…