High-level ab initio and DFT calculations are performed on K+. H2O, K+ (.) N-2 and K+ (.) CO2. We use large basis sets, both all-electron as well as ECP-based basis sets, for potassium. The calculated geometries and harmonic vibrational frequencies are shown to be reliable, with consistent results being obtained, and very good agreement with previous reliable calculations and experiment, where available. The basis set superposition error is shown to be small, and hence we believe that the binding energies, D-0, obtained (16.4, 4.4 and 9.0 kcal mol(-1), respectively) are reliable. (C) 2003 Elsevier Science B.V. All rights reserved.