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F.Y. Naumkin and D.J. Wales
In particular, H 2 encapsulated in small Mg n cluster cages can form weakly bound
or even metastable species, although with a low storage capacity (∼1 weight-%) [8].
In order to try to improve the situation, the present work has two aims: (1) investigate
similar systems of a lighter metal, Be; (2) evaluate feasibility of merging them into
assemblies as a step to material.
Previous relevant work includes modeling adsorption of atomic H on solid Be
(see [9] and references therein). The desorption temperature for molecular hydrogen has been predicted as ∼450 °C, with the desorption energy of ∼1 eV, which
is even higher than for bulk MgH 2 . Another family of systems studied have been
BeH 2 aggregates and “polymers” [10], for which species, however, the hydrogen
desorption temperature or energy has not been specified. The present work employs
the earlier results indicating cage isomers of small Be n clusters as most stable [11].
12.2 Computational Methods and Tools
Calculations have been carried out at the MP2/aug-cc-pvtz level, followed by the
standard counterpoise BSSE correction [12]. This level of theory is employed as implemented in the NWChem ab initio package [13], and is preferred due to capability
to deal reliably with anticipated non-covalent interactions and strong charge-transfer
in the systems studied.
The system geometries have been fully optimized for all atomic coordinates, with
no constraints (for the C 1 symmetry). Vibrational frequencies have been calculated
to verify local minima of energy. The Be n cages have been preoptimized, then hydrogen molecules have been put inside with different orientations, and the system
then reoptimized. The potential energy barriers (e.g. for hydrogen exit from the
cages) were estimated by pulling H in proper direction (e.g. through a gap between
Be atoms): an appropriate Be–H distance was fixed at a series of values and all the
other coordinates were reoptimized for each displacement.
Higher-spin states have been checked to confirm the ground state multiplicity.
Natural charges on atoms have been calculated using the natural bonding orbital
formalism [14].
12.3 Results and Discussion
12.3.1 H 2 @Be n
It is found that H 2 molecule can be trapped inside small Be n clusters starting from
n = 8 [15]. In particular, this and n = 10 cages generally preserve their shapes enveloping the dihydrogen oriented along their symmetry axis. This is different from
the analogous case of Mg 8 cage which changes its shape [8]. Inside the Be 9 cage,
however, the inserted molecule is trapped perpendicular to the original symmetry
F.Y. Naumkin and D.J. Wales
In particular, H 2 encapsulated in small Mg n cluster cages can form weakly bound
or even metastable species, although with a low storage capacity (∼1 weight-%) [8].
In order to try to improve the situation, the present work has two aims: (1) investigate
similar systems of a lighter metal, Be; (2) evaluate feasibility of merging them into
assemblies as a step to material.
Previous relevant work includes modeling adsorption of atomic H on solid Be
(see [9] and references therein). The desorption temperature for molecular hydrogen has been predicted as ∼450 °C, with the desorption energy of ∼1 eV, which
is even higher than for bulk MgH 2 . Another family of systems studied have been
BeH 2 aggregates and “polymers” [10], for which species, however, the hydrogen
desorption temperature or energy has not been specified. The present work employs
the earlier results indicating cage isomers of small Be n clusters as most stable [11].
12.2 Computational Methods and Tools
Calculations have been carried out at the MP2/aug-cc-pvtz level, followed by the
standard counterpoise BSSE correction [12]. This level of theory is employed as implemented in the NWChem ab initio package [13], and is preferred due to capability
to deal reliably with anticipated non-covalent interactions and strong charge-transfer
in the systems studied.
The system geometries have been fully optimized for all atomic coordinates, with
no constraints (for the C 1 symmetry). Vibrational frequencies have been calculated
to verify local minima of energy. The Be n cages have been preoptimized, then hydrogen molecules have been put inside with different orientations, and the system
then reoptimized. The potential energy barriers (e.g. for hydrogen exit from the
cages) were estimated by pulling H in proper direction (e.g. through a gap between
Be atoms): an appropriate Be–H distance was fixed at a series of values and all the
other coordinates were reoptimized for each displacement.
Higher-spin states have been checked to confirm the ground state multiplicity.
Natural charges on atoms have been calculated using the natural bonding orbital
formalism [14].
12.3 Results and Discussion
12.3.1 H 2 @Be n
It is found that H 2 molecule can be trapped inside small Be n clusters starting from
n = 8 [15]. In particular, this and n = 10 cages generally preserve their shapes enveloping the dihydrogen oriented along their symmetry axis. This is different from
the analogous case of Mg 8 cage which changes its shape [8]. Inside the Be 9 cage,
however, the inserted molecule is trapped perpendicular to the original symmetry