Reliable potential for small sulfuric acid-water clusters

We have constructed a reliable potential model for clusters of sulfuric acid and water. Such model requires potentials for sulfuric acid (H₂SO₄), protonated sulfuric acid (HSO₄ ^-), hydronium ion (H₃O⁺) and water (H₂O). To develop this model, we have used the available ab initio data for small clusters containing one sulfuric acid. The ab initio data are well reproduced with our model. We computed the number of waters around a sulfuric acid at atmospheric conditions (300 K and 50% relative humidity) to be ca. 1.3. This is in good agreement with experiments which predict 1.3 waters around the acid. We have also tested our model with small clusters containing two sulfuric acids and find the results to be in good agreement with ab initio calculations. Larger systems with up to 50 water molecules were studied as well. Here, the hydronium ions were found to be on the surface of the cluster and the bisulfate ions inside the cluster. The clusters are very flexible and large fluctuations were observed in the sulfur-sulfur and sulfur-hydronium distance.