The structure and hydrogen bonding in dimer cation systems of H3O+ with some chemical species have been studied using DFT/B3LYP and MP2 with 6-311++G(d,p) basis set, and CCSD/aug-cc-pVTZ methods. The order of binding energies of the studied system, (H3O center dot center dot center dot(A) under barB)(+), as calculated with UCCSD(T)/aug-cc-pVTZ method follow an order: HC (N) under bar > H-2(C) under bar > HO (O) under bar > (O) under barH > (S) under barH > H (F) under bar > H (Br) under bar > H (Cl) under bar > O (N) under bar> N (O) under bar. The structure of (H3O center dot center dot center dot AB)(+) can be described as a hydrogen-bonded complex of H3O+ with AB in which the hydrogen atom from H3O+ makes a hydrogen bond with A-atom of AB species. (C) 2009 Published by Elsevier B. V.