The structure of m-nitrophenyl-1H-pyrrole-2-carboxamide (NPC) and its halide anion complex are optimized by density functional theory B3LYP method at the LANL2DZ level. The conformation change of the host and the supra-molecular interaction between the host and halide anion, in the formation of complex, have been investigated by analyzing the geometric parameters, charge population, frontier orbital, binding energy and thermodynamic parameters. The results show that anion recognition of NPC is spontaneous. Recognition process includes two steps: 1, conformation change of host in the inducement of anion; 2, hydrogen-bonding formation between the activated host and halide anion. The frontier orbital of the host interacts with that of the guest anion, and the electron is transferred from HOMO of halide anion to LUMO of host. Moreover, NPC-F^－ is the most stable among the complex NPC-X^－ (X^－＝F^－, Cl^－, Br^－, I^－). That agrees well with the experimental results of anion recognition of NPC reported in the literature.

Key words: m-nitrophenyl-1H-pyrrole-2-carboxamide, supra-molecular interaction, density functional theory