J. Christian Schön

Computational Design and Prediction of Interesting Not-Yet-Synthesized Structures of Inorganic Materials by Using Building Unit Concepts

The computational design of new and interesting inorganic materials is still an ongoing challenge. The motivation of these efforts is to aid the often difficult task of crystal structure determination, to rationalize different but related structure types, or to help limit the domain of structures that are possible in a given system. Over the past decade, simulation methods have continuously evolved towards the prediction of new structures using minimal input information in terms of symmetry, cell parameters, or chemical composition. So far, this task of identifying candidate structures through an analysis of the energy landscape of chemical systems has been particularly successful for predominantly ionic systems with relatively small numbers of atoms or ions in the simulation cell. After an introductory section, the second section of this work presents the historical developments of such simulation methods in this area. The following sections of the work are dedicated to the introduction of the building unit concept in simulation methods: we present simulation approaches to structure prediction employing both primary (aggregate of atoms) and secondary (aggregate of coordination polyhedra) building units. While structure prediction with primary units is a straightforward extension of established approaches, the AASBU method (automated asssembly of secondary building units) focusses on the topology of network-based structures. This method explores the possible ways to assemble predefined inorganic building units in three-dimensional space, opening the way to the manipulation of very large building units (up to 84 atoms in this work). As illustrative examples we present the prediction of candidate structures for Li(4)CO(4), the identification of topological relationships within a family of metalphosphates, ULM-n and MIL-n, and finally the generation of new topologies by using predefined large building units such as a sodalite or a double-four-ring cage, for the prediction of new and interesting zeolite-type structures.

Stability of Alkali Metal Halide Polymorphs as a Function of Pressure

We investigated the regions of thermodynamic stability of possible modifications of the alkali metal halides as a function of pressure and type of alkali metal and halogen. Both Hartree-Fock and density functional calculations (for six different functionals) were performed. The results are in good agreement with experiment, and the trends in the computed quantities such as transition pressures and lattice parameters as a function of the ab initio method are similar to those found in earlier studies of the alkali metal sulfides. We predict that in most of these systems the so-called 5-5 modification should be metastable at standard pressure and be thermodynamically stable at slightly negative pressures. The sizes of the pressure ranges over which the various modifications are stable showed characteristic trends as a function of the type of the constituent elements, thus generalizing the traditional pressure-homologue rule for transition pressures and stable phases in ionic solids.

Preferential trapping on energy landscapes in regions containing deep-lying minima: The reason for the success of simulated annealing?

Evidence has been accumulating that many complex systems are characterized by energy landscapes that contain many pockets with exponentially growing densities of states. Such pockets possess characteristic trapping temperatures, , such that random (Monte Carlo) walkers on such surfaces will be caught in the pockets, if . It will be shown that the walkers are trapped preferentially in basins containing the deep-lying minima. This observation might serve as an explanation of simulated annealings success to deliver good suboptima in spite of the use of optimization times far less than those times for which convergence of the algorithm has been proven. Furthermore, preferential trapping might also be involved in certain physical processes, e.g. the glass transition.

Sterically Active Electron Pairs in Lead Sulfide? An Investigation of the Electronic and Vibrational Properties of PbS in the Transition Region Between the Rock Salt and the α‐GeTe‐Type Modifications

Recently, we have investigated the energy landscape of PbS for many different pressures on the ab initio level by using Hartree-Fock and density functional theory to globally search for possible thermodynamically stable and metastable structures. The perhaps most fascinating observation was that besides the experimentally known modification exhibiting the rock salt structure a second minimum exists close-by on the landscape showing the low-temperature α-GeTe-type structure. In the present study, we investigate the possible reasons for the existence of this metastable modification; in particular we address the question, whether the α-GeTe-type modification might be stabilized (and conversely the rock salt modification destabilized) by steric effects of the non-bonding electron pair.

Modeling the sol–gel synthesis route of amorphous Si3B3N7

The amorphous high-performance ceramic a-Si3B3N7 can only be synthesized via a sol–gel process. Using a separation of time scales approach, we model various aspects of the sol–gel process explicitly including the fact that the polymerization step occurs via the cross-linking of single-source precursor molecules. For chemically reasonable choices of process parameters, this approach yields satisfactory agreement with the structural data of the amorphous ceramic. In particular, the coexistence of medium range order observed in NMR experiments and the high degree of homogeneity on larger length scales appears to be due to merging many oligomers formed by the linkage of only a few monomers that contain a boron- or silicon-rich core, instead of due to the linking of a few large clusters.

Energy Landscape Exploration of Sub‐Nanometre Copper–Silver Clusters

The energy landscapes of sub-nanometre bimetallic coinage metal clusters are explored with the Threshold Algorithm coupled with the Birmingham Cluster Genetic Algorithm. Global and energetically low-lying minima along with their permutational isomers are located for the Cu(4)Ag(4) cluster with the Gupta potential and density functional theory (DFT). Statistical tools are employed to map the connectivity of the energy landscape and the growth of structural basins, while the thermodynamics of interconversion are probed, based on probability flows between minima. Asymmetric statistical weights are found for pathways across dividing states between stable geometries, while basin volumes are observed to grow independently of the depth of the minimum. The DFT landscape is found to exhibit significantly more frustration than that of the Gupta potential, including several open, pseudo-planar geometries which are energetically competitive with the global minimum. The differences in local minima and their transition barriers between the two levels of theory indicate the importance of explicit electronic structure for even simple, closed shell clusters.

How can Databases assist with the Prediction of Chemical Compounds

An overview is given on the ways databases can be employed to aid in the prediction of chemical compounds, in particular inorganic crystalline compounds. Methods currently employed and possible future approaches are discussed.

Competitive trapping in complex state spaces

In complex state space dynamics at finite time scales, the trapping in certain regions of state space is of great importance, e.g. in the field of protein folding or in the application of stochastic global optimization algorithms. Here, we analyze the influence of the density of states on the features of the trapping process. In particular, we compare the trapping power of a valley with a power-law density of states to one with an exponentially growing density of states. The outcome of this competition crucially depends on the annealing speed and shows that the clear difference between these two paradigmatic densities of states observed at very slow (near-equilibrium) annealing is lost for fast non-equilibrium processes, and that the outcome of the relaxation can strongly depend on the time scale of the process and subtle features of the density of states.

Theoretical and experimental exploration of the energy landscape of the quasi-binary cesium chloride/lithium chloride system.

As a case study, the energy landscape of the cesium chloride/lithium chloride system was investigated by combining theoretical and experimental methods. Global optimization for many compositions of this quasi-binary system gave candidates for possible modifications that constitute promising targets for subsequent syntheses based on solid-state reactions. Owing to the synergetic and complementary nature of the computational and experimental approaches, a substantially better efficiency of exploration was achieved. Several new phases were found in this system, for the compositions CsLiCl(2) and CsLi(2)Cl(3), and their thermodynamic ranking with respect to the already-known phases was clarified. In particular, the new CsLiCl(2) modification was shown to be the low-temperature phase, whilst the already-known modification for this composition corresponded to a high-temperature phase. Based on these results, an improved cesium chloride/lithium chloride phase diagram was derived, and this approach points the way to more rational and more efficient solid-state synthesis.

The threshold algorithm: Description of the methodology and new developments

Understanding the dynamics of complex systems requires the investigation of their energy landscape. In particular, the flow of probability on such landscapes is a central feature in visualizing the time evolution of complex systems. To obtain such flows, and the concomitant stable states of the systems and the generalized barriers among them, the threshold algorithm has been developed. Here, we describe the methodology of this approach starting from the fundamental concepts in complex energy landscapes and present recent new developments, the threshold-minimization algorithm and the molecular dynamics threshold algorithm. For applications of these new algorithms, we draw on landscape studies of three disaccharide molecules: lactose, maltose, and sucrose.