Characteristic features of the positron binding properties for F-(H2O)(n) and Cl-(H2O)(n) (n = 0-3) clusters are elucidated by implementation of the energy gradient of the positron-attached complexes with respect to the nuclear coordinates and positronic basis centers in the scheme of the multi-component molecular orbital calculation. The differences in the positron affinities (PAs) between these clusters are found to decrease with the number of water molecules from n = 0 (1.10 eV) to 3 (0.14 and 0.22 eV for vertical and relaxed PAs, respectively), which is consistent with the changes in the hydrogen-bonded distances, the population on halogen atoms, and the characters of positronic orbitals and electronic HOMOs. (C) 2009 Elsevier B. V. All rights reserved.