Bottom-up and top-down derived nanoparticle structures refined by accurate ab initio calculations are used to investigate the size dependent emergence of crystallinity in titania from the monomer upwards. Global optimisation and data mining are used to provide a series of (TiO2)N global minima candidates in the range N = 1–38, where our approach provides many new low energy structures for N > 10. A range of nanocrystal cuts from the anatase crystal structure are also considered up to a size of over 250 atoms. All nanocrystals considered are predicted to be metastable with respect to non-crystalline nanoclusters, which has implications with respect to the limitations of the cluster approach to modelling large titania nanosystems. Extrapolating both data sets using a generalised expansion of a top-down derived energy expression for nanoparticles, we obtain an estimate of the non-crystalline to crystalline crossover size for titania. Our results compare well with the available experimental results and imply that anatase-like crystallinity emerges in titania nanoparticles of approximately 2–3 nm diameter.

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Predicting size-dependent emergence of crystallinity in nanomaterials: titania nanoclusters versus nanocrystals

New potential energy functions (i-TTM) describing the interactions between halide ions and water molecules are reported. The i-TTM potentials are derived from fits to electronic structure data and include an explicit treatment of two-body repulsion, electrostatics, and dispersion energy. Many-body effects are represented through classical polarization within an extended Thole-type model. By construction, the i-TTM potentials are compatible with the flexible and fully ab initio MB-pol potential, which has recently been shown to accurately predict the properties of water from the gas to the condensed phase. The accuracy of the i-TTM potentials is assessed through extensive comparisons with CCSD(T)-F12, DF-MP2, and DFT data as well as with results obtained with common polarizable force fields for X–(H2O)n clusters with X– = F–, Cl–, Br–, and I–, and n = 1–8. By construction, the new i-TTM potentials will enable direct simulations of vibrational spectra of halide-water systems from clusters to bulk and interf...

i-TTM Model for Ab Initio-Based Ion–Water Interaction Potentials. 1. Halide–Water Potential Energy Functions

Dynamic reconstruction under a physicochemical environment is an intrinsic property of solid surfaces, in particular when associated with catalysis on nanosized or nanostructured materials. Here, we report nonempirical structure determination of TiCl4-capped MgCl2 nanoplates that is based on the combination of a genetic algorithm and density functional calculations. The methodology for the nonempirical structure determination was developed, and its application was demonstrated for 7MgCl2, 15MgCl2, and 15MgCl2/4TiCl4 in relation to the hidden identity of primary particles of the Ziegler–Natta catalyst. Bare MgCl2 nanoplates dominantly exposed {100} surfaces at their lateral cuts, but the chemisorption of TiCl4 induced reconstruction by stabilizing {110} surfaces. The most important finding of the present research is that TiCl4 exhibited distributed adsorption states as consequences of chemisorption on nonideal finite surfaces and the diversity of thermodynamically accessible structures. The assessment of t...

Machine Learning-Aided Structure Determination for TiCl4–Capped MgCl2 Nanoplate of Heterogeneous Ziegler–Natta Catalyst

High-resolution anion photoelectron spectra are reported for the group 4 metal dioxide clusters Ti2O4– and Zr2O4–. Slow photoelectron velocity-map imaging (SEVI) spectroscopy of cryogenically cooled, mass-selected anions yields photoelectron spectra with submillielectronvolt resolution, revealing extensive and well-resolved vibrational progressions. By comparison of the spectra with Franck–Condon simulations, we have identified the C2v and C3v isomers as the ground states of Ti2O4– and Zr2O4– anions, respectively. Minor contributions from the C2h isomer of Ti2O4– and the C2v isomer of Zr2O4– are also seen. The SEVI spectra yield upper bounds for the adiabatic detachment energies, as well as vibrational frequencies for various modes of the neutral Ti2O4 and Zr2O4 species.

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Structural Isomers of Ti2O4 and Zr2O4 Anions Identified by Slow Photoelectron Velocity-Map Imaging Spectroscopy

Three-dimensional models for the aqueous solvation structures of chloride, bromide, and iodide are reported. K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near edge (MXAN) analyses found well-defined single shell solvation spheres for bromide and iodide. However, dissolved chloride proved structurally distinct, with two solvation shells needed to explain its strikingly different X-ray absorption near edge structure (XANES) spectrum. Final solvation models were as follows: iodide, 8 water molecules at 3.60 ± 0.13 A and bromide, 8 water molecules at 3.40 ± 0.14 A, while chloride solvation included 7 water molecules at 3.15 ± 0.10 A, and a second shell of 7 water molecules at 4.14 ± 0.30 A. Each of the three derived solvation shells is approximately uniformly disposed about the halides, with no global asymmetry. Time-dependent density functional theory calculations simulating the chloride XANES spectra following from alternative solvation spheres revealed surprising sensitivity of the electronic state to 6-, 7-, or 8-coordination, implying a strongly bounded phase space for the correct structure during an MXAN fit. MXAN analysis further showed that the asymmetric solvation predicted from molecular dynamics simulations using halide polarization can play no significant part in bulk solvation. Classical molecular dynamics used to explore chloride solvation found a 7-water solvation shell at 3.12 (-0.04/+0.3) A, supporting the experimental result. These experiments provide the first fully three-dimensional structures presenting to atomic resolution the aqueous solvation spheres of the larger halide ions.

Solvation structure of the halides from x-ray absorption spectroscopy

In this article, we propose a stochastic search-based method, namely genetic algorithm (GA) and simulated annealing (SA) in conjunction with density functional theory (DFT) to evaluate global and local minimum structures of (TiO2)n clusters with n = 1-12. Once the structures are established, we evaluate the infrared spectroscopic modes, cluster formation energy, vertical excitation energy, vertical ionization potential, vertical electron affinity, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps, and so forth. We show that an initial determination of structure using stochastic techniques (GA/SA), also popularly known as natural algorithms as their working principle mimics certain natural processes, and following it up with density functional calculations lead to high-quality structures for these systems. We have shown that the clusters tend to form three-dimensional networks. We compare our results with the available experimental and theoretical results. The results obtained from SA/GA-DFT technique agree well with available theoretical and experimental data of literature.

Structural, spectroscopic aspects, and electronic properties of (TiO2)n clusters: a study based on the use of natural algorithms in association with quantum chemical methods.

In this article, we explore the efficiency of using a coupled genetic algorithm (GA) and density functional theory (DFT) based strategy to evaluate probable structures of (H2O)nF− micro-clusters, with n = 1 − 6. We use the stochastic optimization technique of GA to arrive at structures of the cluster systems and once the structures are obtained, do a DFT calculation with the optimized coordinates from the GA calculation as input to get the infra-red spectrum of all the systems. The results of our work closely resembles the pure quantum chemical results obtained by Baik et al. (J Chem Phys 1999, 110, 9116–9127) © 2011 Wiley Periodicals, Inc. J Comput Chem, 2012

Structure and spectroscopy of water-fluoride microclusters: a combined genetic algorithm and DFT-based study.

In this article, we propose a stochastic search based method, namely genetic algorithm in conjunction with density functional theory to evaluate structures of water-halide microclusters, with the halide ion being Cl(-), Br(-), and I(-). Once the structures are established, we evaluate the infrared spectroscopic modes, vertical detachment energies and natural population analysis based charges. We compare our results with available experimental and theoretical results.

Structure and spectroscopic aspects of water‐halide ion clusters: A study based on a conjunction of stochastic and quantum chemical methods

In this manuscript we use a two-tier approach of combining stochastic search strategies and DFT calculations to arrive at extremely good quality structures for neutral (CO2)
 n
 clusters with n = 2–13. The idea of using stochastic search strategy initially is to find out extreme quickly, that is with low computational cost, more than one reasonably good structures by exploring an empirical potential energy surface for CO2 clusters. These pre-optimized structures [using genetic algorithm (GA) as stochastic optimizer] if used as inputs for subsequent DFT calculations using dispersion-corrected ωB97XD and meta hybrid M06-2X functionals give global minimum structures for these systems. This two-step strategy in our opinion is a more potent route to obtain the global minimum structures with certainty rather than a dedicated quantum chemical calculation carried from the outset. We further analyse in detail the structural, vibrational spectra and interaction energies of the clusters studied. We compare our results with experimental and theoretical results available in literature. The correspondence with experimental results is nearly 99 %, that is the results show marginal deviation from reported experimental vibrational frequencies and hence we conclude that GA in conjunction with dispersion-corrected DFT is a viable method for the study of clusters where the interaction is non-covalent in nature. Since B2PLYP-D is also an efficient functional in the study of these systems, we also report interaction energies of the cluster systems examined using this functional.

Structural and spectroscopic studies of carbon dioxide clusters: a combined genetic algorithm and DFT based study

In this article, we explore the feasibility of using stochastic optimization techniques, which are inspired by natural processes, namely simulated annealing (SA) and genetic algorithm (GA) in association with DFT, to find out the global minimum structures of (MgO)
 n
 clusters with n being in the range of 2–15. To check whether the structures are indeed the correct ones, we proceed to do several property calculations like IR-spectroscopic modes, vertical excitation energy, cluster-formation energy, vertical ionization potential, the HOMO–LUMO gap as well as polarizability and hyperpolarizability—both static and dynamic. We emphasize on the point that an initial determination of structure using SA/GA leads to very quick relaxation to structures which are very close to the structures predicted from quantum chemical calculations done from the outsets like DFT. The general pattern of these systems to form beautiful three-dimensional lattice networks is also evident from our study.

Structure, spectroscopy and electronic properties of neutral lattice-like (MgO) n clusters: a study based on a blending of DFT with stochastic algorithms inspired by natural processes

Soumya Ganguly Neogi

Papers

Structural, spectroscopic aspects, and electronic properties of (TiO2)n clusters: a study based on the use of natural algorithms in association with quantum chemical methods.

Structure and spectroscopy of water-fluoride microclusters: a combined genetic algorithm and DFT-based study.

Structure and spectroscopic aspects of water‐halide ion clusters: A study based on a conjunction of stochastic and quantum chemical methods

Structural and spectroscopic studies of carbon dioxide clusters: a combined genetic algorithm and DFT based study

Structure, spectroscopy and electronic properties of neutral lattice-like (MgO) n clusters: a study based on a blending of DFT with stochastic algorithms inspired by natural processes