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A new intermolecular interaction potential for the adenine dimer has been developed with the help of a combination of symmetry-adapted perturbation theory and density functional theory (DFT-SAPT). Supermolecular intermolecular interaction energy computations on hydrogen-bonded and stacked adenine dimers at B3LYP-D, MP2, SCS-MP2, SCS-MI-MP2 and CCSD(T) levels showed that DFT-SAPT is in a very good agreement with CCSD(T). The developed ab initio intermolecular potential has been used to predict the cluster structure of adenine dimers, trimers and tetramers. These global cluster optimizations reproduced adenine dimers reported in the literature and moreover new low-energy structures were also located. For trimers and tetramers, new hydrogen-bonded and stacked low-energy structures have also been found. The current findings suggest that the new ab initio potential can further be exploited to reveal the structure and energy of much larger supramolecular adenine clusters.

We developed a new first principles potential for the cytosine dimer. The ab initio calculations were performed with a DFT-SAPT combination of the symmetry-adapted perturbation method and density functional theory, and fitted to a model site-site functional form. The model potential was used to predict cluster structures up to cytosine hexamers. The global cluster structure optimizations showed that the new potential is able to reproduce some of the 2D filament structures. Moreover many new non-planar cytosine cluster structures were also discovered. Interaction energies of these clusters were compared with B3LYP-D, MP2, SCS-MP2, SCS-MI-MP2 and AMBER. It has been shown that the model agrees well with all ab initio methods, especially for the cytosine hexamer. The model potential outperforms the AMBER force field and therefore it can be exploited to study more challenging larger systems.

Organoclays are very important composite materials used in many diverse industries including plastics and paint. In this study, an organoclay was formed using sepiolite and homologs of quaternary amine surfactants of tetradecyl dimethyl ethylbenzyl ammonium chloride (TDEBAC), hexadecyl dimethyl ethylbenzyl ammonium chloride (HDEBAC) and octadecyl dimethyl ethylbenzyl ammonium chloride (ODEBAC). Adsorption isotherms of sepiolite/quaternary amine surfactants show that the surfactant with the longest chain, ODEBAC, is adsorbed at most onto the surface of sepiolite. FTIR spectroscopy of organosepiolite indicated that spectroscopic features related to the surface and tunnel of sepiolite are notably modified in the presence of surfactants. In addition to these experiments, high level ab initio counter-poise corrected binding energy computations at MP2, SCS-MP2, B97-D and B2PLYP-D levels of theory employing the TZVP basis set were also performed. At all levels, basal interaction of the surfactant at the surface of sepiolite was found to exhibit the most favourable configuration. Furthermore, penetration of the surfactant into the intra-crystalline cavities of sepiolite was also found to be plausible.

Energy storage in the form of ammonia bound in metal salts, so-called metal ammines, combines high energy density with the possibility of fast and reversible NH3 ab- and desorption kinetics. The mechanisms and processes involved in the NH3 kinetics are investigated by density functional theory (DFT) and quasielastic neutron scattering (QENS). The crystal structures of Mg(NH3)nCl2 with n = 6, 2, 1, which contains up to 9.19 wt % hydrogen and 0.115 kg hydrogen L−1, are first analyzed using an algorithm based on simulated annealing (SA), finding all the experimentally known structures and predicting the C2/m structure for the uncharacterized low temperature phase of Mg(NH3)6Cl2. It is found from DFT that the rotation of ammonia in the hexammine complex (n = 6) requires an activation energy of 0.09 eV in the low temperature phase of Mg(NH3)6Cl2 and 0.002–0.12 eV in the high temperature phases; effectively having free rotors as observed experimentally. The findings are supported by the QENS data, which identify C3 rotations of NH3 in the low temperature phase with an activation energy of 0.09 eV. The calculated diffusion rates were found to be 106–107 Hz at the desorption temperatures for all n = 6, 2, 1 systems. DFT calculations involving bulk diffusion of NH3 correctly reproduces the trends observed in the experimental desorption enthalpies. In particular, for n = 6, 2, 1, there is a good agreement between activation barriers and experimental enthalpies. These results indicate that the desorption of NH3 is likely to be diffusion limited.

We have performed global parameter optimization of selected empirical potentials for silicon, resulting in improved performance for small to medium-sized silicon clusters, as judged by a comparison of globally optimized cluster structures to the structures accepted in the literature for the size range up to n = 10. Using global cluster structure optimizations with the resulting optimized model potential and ensuing local optimizations at the DFT level, we could find improved proposals for global minimum structures in the size region n = 10–16. This study confirms the applicability of our general global cluster optimization strategy for still larger silicon clusters.