Association free energies ΔGa are calculated for two different types of host-guest systems, the rigid cucurbit[7]uril (CB7) and the basket shaped octa-acid (OA), and a number of charged guest molecules each by quantum chemical methods from first principles in the context of a recent blind test challenge (SAMPL4). For CB7, the overall agreement between theory and experiment is excellent. In comparison with all other submitted calculated relative ΔGa,rel values for this part of the blind test, our results ranked on top. Modeling the binding free energy in the case of the OA host mainly suffers from the problem that the binding situation is undefined with respect to the charge state and due to its intrinsic flexibility the host-guest complex is not represented well by a single configuration, but qualitative features of the binding process such as the proper binding orientation and the order of magnitude of ΔGa are represented in accord with the experimental expectations even though an accurate ranking is not possible.