The rate constants for concerted exchange reaction of the small cyclic water clusters (H2O)(n)(3, 4) were calculated with the YL method. The anharmonic effect on the concerted hydrogen exchange reaction was examined. For the trimer water, at temperature 300 K, the anharmonic rate constant is k 4: 70 x 10(-4) s(-1), and for the tetramer water, the value is 0: 55 s(-1). The tunneling effect is very small for this two concerted hydrogen exchange reactions and can be neglected in the two reactions. The results are in reasonable agreement with the value given by Loerting et al. Besides that, the anharmonic effect is not significant for the concerted hydrogen exchange reaction of (H2O)(n)(3, 4). (C) 2009 Elsevier B. V. All rights reserved.