Our group recently used CID to probe the stability of atmospherically relevant glycine ammonium bisulfate clusters. The experiments were carried out in a QIT-MS (Thermo LTQ XL). Both bare ammonium bisulfate and glycine ammonium bisulfate clusters exhibited preferential loss of ammonia or an ammoniasulfuric acid pair. The absence of any glycine-loss fragments from the mixed clusters showed that glycine was bound more strongly to sulfuric acid than ammonia. Quantum chemical calculations, however, revealed comparable binding free energies (∆G b ) for glycine, ammonia, and sulfuric acid and that the energy required to remove an ammonia-sulfuric acid pair from a mixed cluster is ≈10 kcal/mol greater than the energy required to remove glycine. This suggests that removing glycine from the cluster is substantially easier, a result that was not experimentally observed. Applying the calculated ∆G b to cluster growth models could lead to substantial errors in the computed growth rates (as rate depends exponentially on ∆G b ). Thus, this work reiterates the need for experimental validation of computed ∆G b .