The equilibrium distribution of arginine molecules in canonical and zwitterionic forms predicted by the theory failed to correctly reproduce the double-peak feature observed in the IR experiment. We have shown from first principles calculations that the energetically favorable dimerization of zwitterions can effectively eliminate the pathway for the structure conversion between two distinct canonical forms and reduce the probability for these canonical forms to reach their thermal equilibrium. The role of dispersion effect is discussed. It suggests that the experimental IR spectrum is determined by the way in which the conformers of arginine are initially generated. (C) 2014 Elsevier B.V. All rights reserved.