The CTTS precursor states in X-(H2O)(n) clusters are studied by the ab initio molecular orbital method. We systematically compare the effects of the solvent structure and anion sizes on the excitation energy from the ground state to the first CTTS precursor state (E-CTTS) and the vertical binding energy (VBE) from the first CTTS precursor state. VBE is found to be more sensitive to solvent environmental changes than ECTTS. At n = 3-5, VBE for a surface ion configuration is ordered as Cl- < Br- < I-, whereas E-CTTS follows the opposite trend. More complex VBE variation is observed for the internal configuration, when n and anions are varied. (C) 2003 Elsevier Science B.V. All rights reserved.