Dynamic and quantum size effects in molecular clusters

The evolution of thermodynamic, dynamic and quantum size effects in clusters is expected to contribute towards the merging between microscopic and macroscopic points of view in molecular, surface and bulk phenomena.     

On bond orders and valences in the Ab initio quantum chemical theory

The concept of bond order and valence indices calculated at the ab initio level is discussed in connection with their close relationship to the nonclassical exchange effects in bonding. An improved definition of bond order and free valence indices is given for the open-shell SCF (UHF ) case, and the generalization of the bond order and valence indices to correlated wave functions is also introduced.     

Valence bond theory for chemical dynamics

This essay provides a perspective on several issues in valence bond theory: the physical significance of semilocal bonding orbitals, the capability of valence bond concepts to explain systems with multireferences character, the use of valence bond theory to provide analytical representations of potential energy surfaces for chemical dynamics by the method of semiempirical valence bond potential energy surfaces (an early example of specific reaction parameters), by multiconfiguration molecular mechanics, by the combined valence bond-molecular mechanics method, and by the use of valence bond states as coupled diabatic states for describing electronically nonadiabatic processes (photochemistry). The essay includes both ab initio and semiempirical approaches.     

Silver clusters in the cages of zeolites: A quantum chemical study

The electronic structure of zeolite A is developed in a step by step procedure from the simple O_hH₈Si₈O₁₂ molecule, to the _∞ ¹ [(-O)₂H₄[Si₈O₁₂)] chain, to the _∞ ² [(-O₄)(Si₈O₁₂)] layer, and finally to the silica zeolite A framework _∞ ³ [Si₂₄O₄₈]. It is remarkable how well the calculated band structures of both, _∞ ² [(-O)₄(Si₈O₁₂)] and _∞ ³ [Si₂₄O₄₈] correspond to the experimentally determined band structure of α-quartz with a Fermi level of -10.55 eV. The HOMO region consists in each case of nonbonding 2p-oxygen bands which in a localized language can be denoted as oxygen lone pairs ( | O<). We observe in each case the typical behaviour of an insulator with saturated valencies whose electronic structure can be described as being localized and is already present in the starting O_h-H₈Si₈O₁₂ molecule. The double-8-rings D8R of the _∞ ² [(-O)₄(Si₈O₁₂)] layer have a pore diameter of 4.1 Å, the same as the pore opening of zeolite A. It is large enough to accept up to four Ag, forming _∞ ² [(-O)₄(Si₈O₁₂)Ag_n], n = 1, 2, 3, 4, layers, suitable for modelling the electronic interactions between the zeolite cavity embedded silver clusters and between the clusters and the zeolite framework. With one Ag per D8R the band structure is simply a superposition of the 4d, 5s and 5p levels of a layer of nearly noninteracting Ag and the silicon dioxide layer. The Ag-d band lies below the oxygen lone pairs, the Ag-s band lies about 3 eV above the oxygen lone pairs, and the Ag-5p bands are in the antibonding silicon dioxide region. The first electronic transition is of oxygen lone pair to Ag-5s LMCT type. Increasing silver content results in progressive splitting of the 5σ Ag bands and shifts the first (Ag^m+ _n)? ← (| O<) charge transfer transition to lower energies. The filled Ag 4d-bands lie always significantly below the (| O<) HOCOs (highest occupied crystal orbitals) but their band width increases with increasing silver content. In all cases the zeolite environment separates the Ag clusters through antibonding Ag-(← O<) interactions so that the coupling remains weak and it makes sense to describe the Ag clusters in the D8R as quantum dots weakly interacting with each other.     

Chemisorption and chemical reactions on size selected clusters

Low energy ion beam techniques have been used to perform a detailed study of the reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ with a range of simple molecules (D₂, CH₄, O₂, C₂H₄, CO and N₂). The reactions were studied over a center of mass collision energy range from 0.2eV up to 7eV. Activation barriers for chemisorption onto the clusters were deduced from the experimental results. The activation barriers for chemisorption on Al ₂₅ ⁺ and Si ₂₅ ⁺ are generally similar and show a qualitative correlation with the electronic properties of the reactant molecule. However, the products of the chemical reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ which result from cluster fragmentation are quite different. Si ₂₅ ⁺ shows a tendency to undergo fission as observed in a number of recent studies of the dissociation of the bare clusters.     

Particle size effects of clusters and crystallites

Size effects of pair distance, cohesive energy, surface stress, and compressibility have been calculated for shell-like structurated particles Y _N ofn complete shells (units Y: rare gases, molecules, ion pairs). The influence of surface definition on size effects has been discussed.     

Hydrogen storage in magnesium clusters: quantum chemical study

Magnesium hydride is cheap and contains 7.7 wt % hydrogen, making it one of the most attractive hydrogen storage materials. However, thermodynamics dictate that hydrogen desorption from bulk magnesium hydride only takes place at or above 300 degrees C, which is a major impediment for practical application. A few results in the literature, related to disordered materials and very thin layers, indicate that lower desorption temperatures are possible. We systematically investigated the effect of crystal grain size on the thermodynamic stability of magnesium and magnesium hydride, using ab initio Hartree-Fock and density functional theory calculations. Also, the stepwise desorption of hydrogen was followed in detail. As expected, both magnesium and magnesium hydride become less stable with decreasing cluster size, notably for clusters smaller than 20 magnesium atoms. However, magnesium hydride destabilizes more strongly than magnesium. As a result, the hydrogen desorption energy decreases significantly when the crystal grain size becomes smaller than approximately 1.3 nm. For instance, an MgH2 crystallite size of 0.9 nm corresponds to a desorption temperature of only 200 degrees C. This predicted decrease of the hydrogen desorption temperature is an important step toward the application of Mg as a hydrogen storage material.     

A subsystem density-functional theory approach for the quantum chemical treatment of proteins

We present an extension of the frozen-density embedding (FDE) scheme within density-functional theory [T. A. Wesolowski and A. Warshel, J. Phys. Chem. 97, 8050 (1993)] that can be applied to subsystems connected by covalent bonds, as well as a practical implementation of such an extended FDE scheme. We show how the proposed scheme can be employed for quantum chemical calculations of proteins by treating each constituting amino acid as a separate subsystem. To assess the accuracy of the extended FDE scheme, we present calculations for several dipeptides and for the protein ubiquitin.     

The secret of dimethyl sulfoxide-water mixtures. A quantum chemical study of 1DMSO-nwater clusters

DMSO-water mixtures exhibit a marked freezing point depression, reaching close to 60 K at n(DMSO) = 0.33. The phase diagram indicates that stable DMSO-water clusters may be responsible for this phenomenon. Using time-independent quantum chemical methods, we investigate possible candidates for stable supermolecules at mole fractions n(DMSO) = 0.25 and 0.33. The model clusters are built by adding various numbers of water molecules to a single DMSO molecule. Structures and interaction energetics are discussed in the light of experimental and theoretical results from the literature. A comparison with results from molecular dynamics simulations is of particular interest. Our optimized structures are spatially very different from those previously identified through MD simulations. To identify the structural patterns characterizing the clusters, we classify them on the basis of hydrogen-acceptor interactions. These are well separated on an interaction energy scale. For the hydrophobic interactions of the methyl groups with water, attractive interactions of up to 8 kJ/mol are found. In forming clusters corresponding to a range of different mole fractions, up to four water molecules are added to each DMSO molecule. This corresponds to a rough local model of solvation. Examination of the trends in the interactions indicates that the methyl-water interaction becomes more important upon solvation. Finally, we investigate how the clusters interact and attempt to explain which role is played by the various structures and their intercluster interaction modes in the freezing behavior of DMSO-water.     

Dynamical mean-field theory from a quantum chemical perspective

We investigate the dynamical mean-field theory (DMFT) from a quantum chemical perspective. Dynamical mean-field theory offers a formalism to extend quantum chemical methods for finite systems to infinite periodic problems within a local correlation approximation. In addition, quantum chemical techniques can be used to construct new ab initio Hamiltonians and impurity solvers for DMFT. Here, we explore some ways in which these things may be achieved. First, we present an informal overview of dynamical mean-field theory to connect to quantum chemical language. Next, we describe an implementation of dynamical mean-field theory where we start from an ab initio Hartree-Fock Hamiltonian that avoids double counting issues present in many applications of DMFT. We then explore the use of the configuration interaction hierarchy in DMFT as an approximate solver for the impurity problem. We also investigate some numerical issues of convergence within DMFT. Our studies are carried out in the context of the cubic hydrogen model, a simple but challenging test for correlation methods. Finally, we finish with some conclusions for future directions.     

Molecular shape effects and quantum theory

Certain features of the chemist's molecular structure model, viz. size and shape, are retrieved even in the best non-adiabatic variational calculations thus far carried out for ground states of H ₂ ⁺ and H₂. Those features do not conflict with the full symmetry of exact molecular eigenstates, once they are properly understood as correlation effects.     

Hydrocarbon adsorption on gold clusters: Experiment and quantum chemical modeling

Heats of adsorption Q of n-alkanes C₆–C₉ on ZrO₂ modified with gold and nickel nanoparticles were determined experimentally. The Q values were found to be higher on average by 7 kJ/mol on the modified samples as compared to the pure support. Density functional theory with the PBE functional and the pseudopotential for gold effectively allowing for relativistic corrections was used to model the adsorption of saturated hydrocarbons on Au and Au + Ni, as exemplified by the interaction of alkanes C₁–C₃ with Au _m , Au _{m − 1}Ni (m = 3, 4, 5) clusters as well as the interaction of C₁–C₈ with Au₂₀. Based on the calculation results, the probable coordination centers of alkanes on nanoparticle surfaces were found to be vertices and edges, whereas face localization was less probable.     

Analysis of the delocalization in the topological theory of chemical bond

The topological analysis of the gradient field of the electron localization function provides a convenient theoretical framework for the partition of the molecular space into basins of attractors having a clear chemical meaning. The basin populations are evaluated by integrating the one-electron density over the basins. The variance of the basin population provides a measure of the delocalization. The behavior of the core C(X) and protonated valence basins V(X, H) populations were investigated. The analysis of the population variance in terms of cross-contributions is presented for aromatic and antiaromatic systems, hypervalent molecules and hydrogen-bonded complexes. For hypervalent molecules this analysis emphasizes the importance of the ionic resonance structures.     

Making and breaking of small water clusters: A combined quantum chemical and molecular dynamics approach

We present a combined quantum chemical and molecular dynamics study of cyclic and noncyclic water n-mers ([(H₂O]_n, n = 2–6) at four different temperatures and showcase that the dynamics of small water clusters can reproduce the known properties of bulk water reasonably well. We investigate the making and breaking of the water clusters by computing the hydrogen bond strengths, average lifetimes, and relative stabilities, which are important to understand the complex solution dynamics. We compare the behavior of water clusters in the gas phase and in the solution phase as well as the variation in the properties as a function of cluster size and highlight the notably more interesting cluster dynamics of the water trimer when compared to the other water clusters. © 2019 Wiley Periodicals, Inc.     

Pressure and size effects in endohedrally confined hydrogen clusters

Density functional theory is used to carry out a systematic study of zero-temperature structural and energy properties of endohedrally confined hydrogen clusters as a function of pressure and the cluster size. At low pressures, the most stable structural forms of (H(2))(n) possess rotational symmetry that changes from C(4) through C(5) to C(6) as the cluster grows in size from n=8 through n=12 to n=15. The equilibrium configurational energy of the clusters increases with an increase of the pressure. The rate of this increase, however, as gauged on the per atom basis is different for different clusters sizes. As a consequence, the size dependencies of the configurational energies per atom at different fixed values of pressure are nonmonotonic functions. At high pressures, the molecular (H(2))(n) clusters gradually become atomic or dominantly atomic. The pressure-induced changes in the HOMO-LUMO gap of the clusters indicate a finite-size analog of the pressure-driven metallization of the bulk hydrogen. The ionization potentials of the clusters decrease with the increase of pressure on them.     

Comments on the status of the chemical bond in methods of quantum chemistry

Chemists do not take the view that chemistry can be derived from quantum mechanics by computations too literally; the difficulties are illustrated by the double bond concept. Quantum mechanics is very useful, but recent calculations have been mainly concerned with physical properties rather than chemical processes. In general, the chemical formula plays a central role in all interpretation of quantum-chemical results, thus suggesting that the chemical bond is a preexistent notion. Moreover, the Born–Oppenheimer approximation seems to be necessary for deriving the existence of bonds between atoms previously assigned to suitable positions. Recent analyses by Claverie and Diner, Woolley, Primas, and others are briefly recalled, concluding with an expression of hope that use of more general Hamiltonians can lead to progress in obtaining a fully independent quantum theory of chemistry.     

Small atomic clusters: quantum chemical research of isomeric composition and physical properties

Structural Chemistry - In this review, we give a brief summary of methodological and computational aspects of determination of structure and different size-dependent properties of small atomic...