(a) Bond dissociation profile of . REAXFF gives an equilibrium bond length of 1.6 Å. This is in excellent agreement with DFT value of 1.59 Å. (b) H–Al–H angle bend in molecule. The energies are computed with reference to the equilibrium angle energy.
Small representative , , clusters used in the training set of REAXFF.
The various polymorphic modifications of illustrated by the connection of the octahedra and channels through the polymorphs.
EoSs for phases (DFT values are drawn using straight lines while those for REAXFF are drawn using dotted lines).
(i) Dissociation profile of as calculated by DFT using nudged elastic band method. (ii) The energy profile during a MD simulation of a heating run of . The temperature was ramped up at a rate of 0.000 25 K/iteration.
Illustrations of the atomic configurations and energy profiles for (a) alane dimerization reaction and (b) agglomeration of molecules.
Snapshots of the clusters at the (a) beginning and (b) end of the simulation.
(a) The completely agglomerated clusters at a temperature of 800 K. No molecular hydrogen was desorbed at this temperature. (b) The Al–Al pair distribution function of the cooled agglomerated clusters. The agglomerate was annealed to 0 K at a rate of 0.0025 K/iteration.
The charge distribution plots of the alane clusters (0 ps) at the beginning of the simulation and (500 ps) that of the agglomerated cluster at the end of the simulation run.
Desorption energy as a function of number of molecules abstracted from the system. The reference energy, shown by the dotted line, is the energy for .
Geometries of the annealed clusters of , , , and . In the hydrogen deficient cluster the hydrogen atoms prefer to occupy surface sites rather than bulk.
Charge distribution plots showing the transfer of charge during abstraction process of molecular hydrogen from cluster. The hydrogen atoms are negatively charged.
The arrow shows a hydrogen molecule trapped in channel.
Bond energy and bond-order parameters. is in kcal/mol.
Atom parameters ( is in kcal/mol).
Valence angle parameters.
Adsorption energies of hydrogen atoms on the high symmetry sites on Al(111) surface. The energies are in kcal/mol per H.
Binding energies of , , and on Al(111) surface.
Binding energies BEs (in kcal/mol ) of small clusters used in the training set.
Comparison between DFT and binding energies BEs of Kawamura et al. (in kcal/mol H) of small clusters.
Relative stability of three phases with respect to the phase using the PAW and US-PP and REAXFF. The units are in kcal/mol.
The temperature at which molecular hydrogen is released from cluster. As the size of the cluster increases the temperature at which molecular hydrogen is released from the cluster also decreases.
The heat of fragmentation of into various clusters during thermal heating of the cluster. The DFT values were computed using VASP at the PW91 level of theory. The energies are in kcal/mol.
The energy of agglomeration (per ) of various small clusters of the series as calculated using DFT and REAXFF.
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