Adem Tekin

How accurate is the density functional theory combined with symmetry-adapted perturbation theory approach for CH–π and π–π interactions? A comparison to supermolecular calculations for the acetylene–benzene dimer

Five different orientations of the acetylene-benzene dimer including the T-shaped global minimum structure are used to assess the accuracy of the density functional theory combined with symmetry adapted perturbation theory (DFT-SAPT) approach in its density-fitting implementation (DF-DFT-SAPT) for the study of CH-pi and pi-pi interactions. The results are compared with the outcome of counterpoise corrected supermolecular calculations employing second-order Møller-Plesset (MP2), spin-component scaled MP2 (SCS-MP2) and single and double excitation coupled cluster theory including perturbative triple excitations (CCSD(T)). For all considered orientations MP2 predicts much deeper potential energy curves with considerably shifted minima compared to CCSD(T) and DFT-SAPT. In spite of being an improvement over the results of MP2, SCS-MP2 tends to underestimate the well depth while DFT-SAPT, employing an asymptotically corrected hybrid exchange-correlation potential in conjunction with the adiabatic local density approximation for the exchange-correlation kernel, is found to be in excellent agreement with CCSD(T). Furthermore, DFT-SAPT provides a detailed understanding of the importance of the electrostatic, induction and dispersion contributions to the total interaction energy and their repulsive exchange corrections.

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

A multifaceted approach to hydrogen storage

The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.

First principles potential for the acetylene dimer and refinement by fitting to experiments

We report the definition and refinement of a new first principles potential for the acetylene dimer. The ab initio calculations were performed with the DFT-SAPT combination of symmetry-adapted intermolecular perturbation method and density functional theory, and fitted to a model site-site functional form. Comparison of the calculated microwave spectrum with experimental data revealed that the barriers to isomerization were too low. This potential was refined by fitting the model parameters in order to reproduce the observed transitions, an excellent agreement within ~1 MHz being achieved.

Global geometry optimization of small silicon clusters with empirical potentials and at the DFT level

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.

Ammonia dynamics in magnesium ammine from DFT and neutron scattering

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.

GLOBAL GEOMETRY OPTIMIZATION OF SILICON CLUSTERS EMPLOYING EMPIRICAL POTENTIALS, DENSITY FUNCTIONALS, AND AB INITIO CALCULATIONS

Sin clusters in the size range n = 4–30 have been investigated using a combination of global structure optimization methods with DFT and ab initio calculations. One of the central aims is to provide explanations for the structural transition from prolate to spherical outer shapes at about n = 25, as observed in ion mobility measurements. Firstly, several existing empirical potentials for silicon and a newly generated variant of one of them were better adapted to small silicon clusters, by global optimization of their parameters. The best resulting empirical potentials were then employed in global cluster structure optimizations. The most promising structures from this stage were relaxed further at the DFT level with the hybrid B3LYP functional. For the resulting structures, single point energies have been calculated at the LMP2 level with a reasonable medium-sized basis set, cc-pVTZ. These DFT and LMP2 calculations were also carried out for the best structures proposed in the literature, including the most recent ones, to obtain the currently best and most complete overall picture of the structural preferences of silicon clusters. In agreement with recent findings, results obtained at the DFT level do support the shape transition from prolate to spherical structures, beginning with Si26 (albeit not completely without problems). In stark contrast, at the LMP2 level, the dominance of spherical structures after the transition region could not be confirmed. Instead, just as below the transition region, prolate isomers are obtained as the lowest-energy structures for n ≤ 29. We conclude that higher (probably multireference) levels of theoretical treatments are needed before the puzzle of the silicon cluster shape transition at n = 25 can safely be considered as explained.

Lithium dihydroborate: first-principles structure prediction of LiBH2.

We report a first-principles structure prediction of the LiBH(2), which structures are modeled by using four formula units per unit cell without symmetry restrictions. The computational methodology combines a simulated annealing approach and density functional total energy calculations for crystalline solid structures. The predicted lowest energy structure shows the formation of linear anionic chains, (∞)(1)[BH(2)], enthalpy of formation at 0 K equal to -90.07 kJ/mol. Ring structures, in particular with butterfly and planar square topologies, are found to be stable but well above the ground state by 20.26 and 12.92 kJ/mol, respectively. All convergent structures fall in the symmetry families monoclinic, tetragonal, and orthorhombic. For the representative structures of each family group, simulated X-ray diffraction patterns and infrared spectra are reported.

Adsorption of quaternary amine surfactants and their penetration into the intracrystalline cavities of sepiolite

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.

Towards a Spectroscopic and Theoretical Identification of the Isolated Building Blocks of the Benzene–Acetylene Cocrystal

Isomer- and mass-selective UV and IR-UV double resonance spectra of the BA3, B2A, and B2A2 clusters of benzene (B) and acetylene (A) are presented. Cluster structures are assigned by comparison with the UV and IR spectra of benzene, the benzene dimer, as well as the BA, BA2, and B2A clusters. The intermolecular vibrations of BA are identified by dispersed fluorescence spectroscopy. Assignment of the cluster structures is supported by quantum chemical calculations of IR spectra with spin-component scaled second-order Møller-Plesset (SCS-MP2) theory. Initial propositions for various structures of the BA3 and B2A2 aggregates are generated with model potentials based on density functional theory combined with the symmetry-adapted perturbation theory (DFT-SAPT) approach. Shape and relative cluster stabilities are then confirmed with SCS-MP2. T-shaped geometries are the dominant structural motifs. Higher-energy isomers are also observed. The detected cluster structures are correlated with possible cluster formation pathways and their role as crystallization seeds is discussed.