In this article we show how to calculate free energies for atmospherically relevant complexes when multiple conformers and/or isomers are present. We explain why the thermal averaging methods used in several published works are incorrect. On the basis of our two sample cases, the sulfuric acid-pinic acid complex and the (H2SO4)(3)(NH3)(3)(H2O)(4) cluster, we provide numerical evidence that the use of these incorrect formulas can result in errors larger than 1 kcal/mol. We recommend that if vibrational frequencies and thus Gibbs free energies of the individual conformers are unavailable, one should not attempt to correct for the presence of multiple conformers and instead use only the global minimum conformers for both reactants and products. On the contrary, if the free energies for the conformers are calculated for both reactants and products, their effect can be accounted for by the statistical mechanical methods presented in this article.