5d-metal mononitrides and monoborides viz. X-N and X-B (X = La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg) are studied using density functional method based B3LYP functional with LANL2DZ and SDD basis set. The lowest spin state, electron affinities, ionization potentials and binding energies for mononitrides and monoborides are obtained. The electronic state and electronic configuration of mononitrides and monoborides are discussed. Orbitals involved in bond formation are identified. The properties of mononitrides and monoborides are compared. It is found that 5d-metal atoms form stronger bond with nitrogen atom than the boron atom. The range of binding energy, electron affinity and ionization potential is wider for mononitrides than that for monoborides. The properties of 5d-metal mononitrides and 3d-metal mononitrides are also compared. The binding energies for the former are lower than those for the latter.

The hydrogen storage capacity of C 2 H 4 V organometallic complex and its cation was obtained using second order Møller-Plesset (MP2) and density functional theory methods with different exchange and correlation functionals. A maximum of five and six hydrogen molecules can be adsorbed on C 2 H 4 V and C 2 H 4 V + compounds with gravimetric H 2 uptake capacities of 11.32 and 13.28 wt %, respectively. The effect of temperature and different exchange and correlation functionals on the adsorption energy of C 2 H 4 V(5H 2 ) and C 2 H 4 V + (6H 2 ) complexes was studied. The superiority of C 2 H 4 V + as a possible hydrogen storage material over C 2 H 4 V and other vanadium containing organometallic compounds was observed. Similar to isolated C 2 H 4 V and C 2 H 4 V + , after adsorption of a maximum of H 2 molecules on C 2 H 4 V and C 2 H 4 V + also, V as well as V + remain strongly bound to the C 2 H 4 substrate. Many-body analysis was carried out to study the nature of interactions between different molecules in a complex, and the contribution from different many-body energies to the binding energy of a complex.

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