Benchmarking of ONIOM method for the study of NH3 dissociation at open ends of BNNTs

The reliability of ONIOM approach have been examined in calculations of adsorption energies, transition structures, change of HOMO-LUMO energy gaps and equilibrium geometries of the interaction between NH3 and N-enriched (A) or B-enriched (B) open ended boron nitride nanotubes. To these ends, four models of the A or B, with different inner and outer layers have been studied. In addition, various low-levels including, AM1, PM3, MNDO and UFF have been examined, applying B3LYP/6-31 G* in all high-levels. It was shown, that in the case of A, (choosing two atom layers of the tube open-end as inner layer) the results of ONIOM approach are in best agreement with those of the pure density functional theory (DFT) calculations, while their results significantly differ from those of DFT in the case of B in same conditions. All above and population analysis demonstrate that the ONIOM may be a reliable scheme in the study of weak interactions while it is a controversial approach and should be applied cautiously in the case of strong interactions. We also probed the effect of tube length and diameter on the consistency between ONIOM and DFT results, showing that this consistency is independent of the mentioned parameters.