Utilizing a comprehensive test set of 205 clusters of atmospheric relevance, we investigate how different DFT functionals (M06-2X, PW91, omega B97X-D) and basis sets (6-311++G(3df,3pd), 6-31++G(d,p), 6-31+G(d)) affect the thermal contribution to the Gibbs free energy and single point energy. Reducing the basis set used in the geometry and frequency calculation from 6-311++G(3df,3pd) --> 6-31++G(d,p) implies a significant speed-up in computational time and only leads to small errors in the thermal contribution to the Gibbs free energy and subsequent coupled cluster single point energy calculation. (C) 2014 Elsevier B.V. All rights reserved.