Basin hopping in conjunction with second order Moller-Plesset perturbation theory is used to characterise the lowest energy isomers of mixed component radical cation clusters of the form [H2O-X](center dot+), [(H2O)(2)-X](center dot+) and [H2O-X-2](center dot+), where X = PH3, H2S and HCl, with the relative energies refined using coupled cluster theory calculations. For the dimers where X = H2S or HCl, a proton transfer based structure comprising H3O+ and SH center dot or Cl-center dot radicals has the lowest energy structure whereas for X = PH3 a hemibonded structure is most stable. For the trimers, a much wider range of possible isomers based upon both proton transfer and hemibonded structural motifs is observed. (C) 2014 Elsevier B.V. All rights reserved.