Ab Initio Study of Magnesium and Magnesium Hydride Nanoclusters and Nanocrystals: Examining Optimal Structures and Compositions for Efficient Hydrogen Storage

On the basis of the attractive possibility of efficient hydrogen storage in light metal hydrides, we have examined a large variety of Mg_<i>n</i>H_<i>m</i> nanoclusters and (MgH₂)_<i>n</i> nanocrystals (<i>n</i> = 2–216, <i>m</i> = 2–436) using high level coupled cluster, CCSD­(T), <i>ab initio</i> methods, and judicially chosen density functional calculations of comparable quality and (near chemical) accuracy. Our calculated desorption energies as a function of size and percentage of hydrogen have pinpointed optimal regions of sizes and concentrations of hydrogen which are in full agreement with recent experimental findings. Furthermore, our results reproduce the experimental desorption energy of 75.5 kJ/mol for the infinite system with remarkable accuracy (76.5 ± 1.5 kJ/mol)

Fully Hydrogenated Beryllium Nanoclusters

We present the ground state and energetically low structures of Be_<i>n</i>H_2<i>n</i> nanoclusters as predicted using density functional theory (DFT) and employing the M06 meta-hybrid exchange-correlation functional. Results using the M06 functional are benchmarked against high accuracy coupled-cluster CCSD­(T) and found to be in excellent agreement. For small values of <i>n</i>, the linear or polymeric form is the lowest energy geometry, while for sizes larger, <i>n</i> > 9 ring type and link type structures are the energetically lowest configurations. This trend has also been observed through ab initio molecular dynamics (AIMD) simulations at finite temperatures. In addition to the binding energies of the structures we report on polymerization energies, Be–H bond energies with respect to coordination details, hydrogen desorption energies of saturated and oversaturated species, as well as computed infrared spectra of all the ground state and energetically low lying structures presented. Furthermore, we find that the saturated polymeric forms of the nanoclusters <i>cannot</i> retain molecular hydrogen, in contrast to what is expected when zero point energy corrections are not taken into account