A Comparative Study of B₂ and Al₂ Using Density functional Methods and Ab Initio Methods

In order to evaluate the ability of the density functional methods on predicting the small molecules, the equilibium geometries and dissociation energies of B₂ and Al₂ were systematically studied with 10 kinds of density functional methods. The results show that the wB97x functional, M06 functional, and its derivatives M06-2X, M06-L, M06-HF cannot predicte the correct ground electronic structures and the dissociation energies of Al₂ molecule. Although the correct ground electronic structure of B₂ was predicted with the density functional methods, the dissociation energies predicted by most of the chosen density function methods are not accurate enough comparing with the experimental values. However, the ab initio methods CCSD(T)/cc-pVTZ和FCI/cc-pVTZ gave the right ground and first excited states of these two molecules, namely, the ground state X³Σ_g^- and the first excited state A³Π_u of B₂, and the ground state X³Π_u and the first excited state A³Σ_g^- of Al₂, which is consistent with the experimental results. The bond distances and the dissociation energies predicted with the two ab initio methods are very comparable with the experiments values.