A density functional method (B3P86) with 6-311＋G(2DF) basis has been used to study the equilibrium geometry, HOMO energy level, LUMO energy level, energy gap and harmonic frequency of aluminium nitride (AlN) ground state molecule under different external electric field ranging from －0.03 to 0.03 a.u. The results show that the magnitude and direction of the external electric field have important effects on these characteristics of the AlN molecule. The bond length and energy gap were found to decrease, but the harmonic frequency, HOMO energy level and LUMO energy level were found to increase with increasing the external field. The maximum HOMO energy level and LUMO energy level were －0.2776 and －0.0828 a.u. at F＝0.01 a.u, respectively. Further increasing the electric field would make the HOMO energy level and LUMO energy level decreased. As the electric field increased to 0.02 a.u., the total molecule energy reached the maximum －297.4217 a.u. Further increasing the electric field resulted in a decrease of the total energy.

Key words: aluminium nitride, external electric field, energy gap, equilibrium geometry