The potential energy surfaces of various species of aspartic acid have been computed at the DFT/B3LYP/6-31G(d) level of theory. Conformations of 34, 11, 9 and 3 were found from 324, 162, 81 and 81 possible conformers for species H 3 asp + , H 2 asp, Hasp − and asp 2− , respectively. The most stable conformation for species H 3 asp + , H 2 asp, Hasp − and asp 2− are γ D , α L , ε L and β L conformers, respectively. Three protonation steps due to the tetrahydrated forms of species asp 2− , Hasp − and H 2 asp of aspartic acid computed at the B3LYP/6-31+G(d, p) level of theory with zero-point vibrational energy corrections are exothermic reactions and their stabilization energies of the stepwise protonation are −244.68, −308.57 and −379.97kcalmol −1 , respectively.