The M(H2O)(19-21)(+) clusters (M = Na or K in cavity) consisting of broken and distorted dodecahedral cages are studied by op timizing geometry at the B3LYP/6-311++G(**) level. The stabilization energy (relative to separated H2O and M+) per monomer (SEP) exhibits a maximum for K(H2O)(+) and no such maximum for Na(H2O)(+) cluster. While K in dodecahedral cavity carries a + 1 charge, Na remains as a neutral atom, and suggests that the electron affinity (EA) of Na+ > EA of (H2O)(+)(20) dodecahedral cage > EA of K+. On the basis of above trends in the SEP values and the charge on the metal, one can explain the absence of a magic number peak for Na(H2O)(+) and the presence of the magic peak for K(H2O)(20)(+) cluster. (C) 2004 Elsevier B.V. All rights reserved.