We present a new systematic configurational sampling algorithm for investigating the potential energy surface of hydrated atmospheric molecular clusters. The algorithm is based on creating a Fibonacci sphere around each atom in the cluster and adding water molecules to each point in nine different orientations. For the sampling of water molecules to existing hydrogen bonds, the cluster is displaced along the hydrogen bond, and a water molecule is placed in between in three different orientations. Generated redundant structures are eliminated based on minimizing the root-mean-square distance of different conformers. Initially, the clusters are sampled using the semiempirical PM6 method and subsequently using density functional theory (M06-2X and ωB97X-D) with the 6-31++G(d,p) basis set. Applying the developed algorithm, we study the hydration of sulfuric acid with up to 15 water molecules. We find that the addition of the first four water molecules "saturate" the sulfuric acid molecule and that they are more thermodynamically favorable than the addition of water molecules 5-15. Using the large generated set of conformers, we assess the performance of approximate methods (ωB97X-D, M06-2X, PW91, and PW6B95-D3) in calculating the binding energies and assigning the global minimum conformation compared to high level CCSD(T)-F12a/VDZ-F12 reference calculations. The tested DFT functionals systematically overestimate the binding energies compared to coupled cluster calculations, and we find that this deficiency can be corrected by a simple scaling factor.

Using computational methods we study the gas phase hydration of three different atmospherically relevant organic acids with up to 10 water molecules. We study the dicarboxylic acid (pinic acid) and a tricarboxylic acid (3-methyl-1,2,3-butanetricarboxylic acid (mbtca)) that are both identified as products from α-pinene oxidation reactions. We also study an 2-oxohexanediperoxy acid (ohdpa) that have been identified as a product from cyclohexene autoxidation. To sample the cluster structures, we employ our recently developed systematic hydrate sampling technique and identify a total of 551 hydrate clusters. The cluster structures and thermochemical parameters (at 298.15 K and 1 atm) are obtained at the ωB97X-D/6-31++G(d,p) level of theory and the single point energy of the clusters have been refined using a high level DLPNO-CCSD(T)/augcc-pVTZ calculation. We find that all three tested organic acids interact significantly weaker with water compared to the primary nucleation precursor sulfuric acid. Even at 100% relative humidity (298.15 K and 1 atm), we find that ohdpa remains unhydrated and only the monohydrate of pinic acid and mbtca are slightly populated (4% and 2%, respectively). From the obtained molecular structures potential implications for ice nucleating ability of aerosol particles is discussed. 2

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Ceria is a promising material for cathodes in hightemperature CO 2 electrolysis cells because ceria can become a mixed electronic and ionic conductor through doping, which enables a high surface area for electrocatalysis. Here, we systemically investigate the effect of strain to enhance the activity for electrocatalytic CO 2 RR on CeO 2 (111) using density functional theory corrected for on-site Coulomb interactions (DFT + U). We find that tensile strain decreases the oxygen vacancy formation energy due to a downshift of the Ce 4f orbital energy, in agreement with the larger size of the Ce 3+ ion than the Ce 4+ ion. The corresponding upshift in the Ce f-band center with compressive strain destabilizes the formation energy of the critical surface oxygen vacancies and reduces the energetic span of the reduction reaction, leading to a 4 orders of magnitude higher turnover frequency at 800 K for 4% compressive strain. These findings shed new light on possible pathways to enhance the catalytic activity for CO 2 RR on CeO 2 (111) and related catalytic systems by strain engineering.

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