J. C. Schön

Determination of symmetries and idealized cell parameters for simulated structures

A robust algorithm is presented that determines the symmetries present in an atomic configuration and idealizes the cell parameters according to the crystal system suggested by the symmetries detected. No information besides the coordinates of the atoms within some arbitrary unit cell of the crystal is required.

Emergent Hierarchical Structures in Complex-System Dynamics

A method is introduced for studying thermal relaxation in multiminima energy landscapes. All the configurations connected to a given energy minimum by paths never exceeding a chosen energy lid are found, each equipped with a set of pointers to its neighbours. This information defines a phase space pocket around the minimum, in which the master equation for the relaxation process is directly solved. As an example we analyse some instances of the Travelling-Salesman Problem. We find that i) the number of configurations accessible from a given suboptimal tour grows exponentially with the energy lid, ii) the density of states within the pocket also shows exponential growth, iii) the low-temperature dynamical behaviour is characterized by a sequence of local equilibrations in increasingly larger regions of phase space and finally iv) the propagator decays algebraically with a temperature-dependent exponent. These observations are related to both theoretical models and experimental findings on relaxation in complex systems.

SIMULATION OF THERMAL CONDUCTIVITY AND HEAT TRANSPORT IN SOLIDS

Using molecular dynamics (MD) with classical interaction potentials we present calculations of thermal conductivity and heat transport in crystals and glasses. Inducing shock waves and heat pulses into the systems we study the spreading of energy and temperature over the configurations. Phonon decay is investigated by exciting single modes in the structures and monitoring the time evolution of the amplitude using MD in a microcanonical ensemble. As examples, crystalline and amorphous modifications of Selenium and

A New Algorithm for Space-Group Determination

\rm{SiO_2}

Structure prediction of high-pressure phases for alkali metal sulfides

are considered.

Determination, prediction, and understanding of structures, using the energy landscapes of chemical systems – Part II

An important part of the crystallographic description of crystal structures, whether they belong to synthesized compounds or have been generated by computer, is the assignment of the correct space group. Since this task often proves to be highly nontrivial, we have developed an algorithm which determines the space group and the transformation to the standard setting of a given crystal structure, where no restrictions are placed on the original description of the structure.

CMPZ– an algorithm for the efficient comparison of periodic structures

For a given chemical system we present a systematic approach to predict structures, which may exist at high pressure, by investigating the global enthalpy landscape. We combine global optimizations, based on empirical potential energy functions, and local optimizations (volume, cell shape, and atomic positions) on both Hartree-Fock and density functional theory level. We predict the existence of high-pressure phases for the alkali metal sulfides of the composition M2S (M = Li, Na, K, Rb, Cs), together with the transition pressures among these phases.

Determination of candidate structures for simple ionic compounds through cell optimisation

Abstract In the past decade, new theoretical approaches have been developed to determine, predict and understand the struc-ture of chemical compounds. The central element of these methods has been the investigation of the energy landscape of chemical systems. Applications range from extended crystalline and amorphous compounds over clusters and molecular crystals to proteins. In this review, we are going to give an introduction to energy landscapes and methods for their investigation, together with a number of examples. These include structure prediction of extended and mo-lecular crystals, structure prediction and folding of proteins, structure analysis of zeolites, and structure determination of crystals from powder diffraction data.

Prediction, determination and validation of phase diagrams via the global study of energy landscapes

The systematic comparison of the atomic structure of solid compounds has become an important task in crystallography, chemistry, physics and materials science, in particular in the context of structure prediction and structure determination of crystalline solids. In this work, an efficient and robust algorithm for the comparison of periodic structures is presented, which is based on the mapping of the point patterns of the two structures into each other. This algorithm has been implemented as the module CMPZ in the structure visualization and analysis program KPLOT.

Structure prediction based on ab initio simulated annealing for boron nitride

Abstract We show at the example of several simple ionic compounds (NaCl, CsCl, LiF, NaF, KF, RbF, CsF, CaO, MgO, K 2 O, Li 2 O, MgF 2 , CaF 2 , Al 2 O) that the global minimisation of the energy through variation of both the basis vectors of the simulation cell and the relative positions of the atoms within the cell leads to structures that are good candidates for the actual structure of the material. During this optimisation, no restrictions were imposed on size, shape, or symmetry of the simulation cell. We investigate in detail the influence of the parameters in the effective interaction potential on the distribution of the structures.