Ab initio geometries, electronic structures of MgB₂ molecule

Ab initio methods with 6-311G* and cc-pVTZ basis sets have been used to calculate the equilibrium geometries and vibrational properties of nine states of MgB₂ molecule. The present computational results predict that the ground state is the singlet state ¹A₁ of T-shaped C_2v (B-Mg-B) equilibrium geometry. The energy of the triplet state ³B₁ of this geometry is only 421 cm^-1 higher than that of the ground state at CCSD(T)/cc-pVTZ level although 2935 cm ^-1 lower at QCISD/6-311G* level. This indicates that the electron correlation effect is very important and it is difficult to distinguish these two states only by their total energies. However, the calculated harmonic vibrational frequencies and the bond length of these two states are obviously different and they may be used to identify the ground state ¹A ₁ from state ³B₁. The dissociation channel MgB₂⇒Mg+B₂ is considered as the preferred one for the ground state X¹A₁ according to the dissociation energies of different channels.