Proton transfer potentials in ammonia complex H3N…H^+…NH3 are calculated by using the quantum chemical ab initio method (4-31G basis set). It shows that the variations of H-bond length and its orientation make no impact on the characteristics of double potential well. The longer H-bond length and the more heavily the molecular main axle deviated from H-bood axle, the higher the energy barrier between double potential wells. Suitable analytic functions can be found with least square fitting to the ab initio results so as to extend the analytic functions' usage range. Considering the system's symmetry and unsymmetry, we modify Φ4 functions. So the simulation results are noticeably improved, and the highest fitting correlation coefficient can reach 0.999.

Key words: AMMONIA, AB INITIO CALCULATION, PROTON MIGRATION