"Future developments in the study of structure-property relations by computational crystallography and experiments"
This workshop (held in Lyon at CECAM, from October 23 to 27, 2000) was planned in order to provide the opportunity to discuss recent advances in theoretical and experimental approaches of interest to crystallographers. It was intended to continue the sequence of workshops on similar topics funded by CECAM (1997) and the ESF (1998). The present workshop would not have been possible without the generous funding provided by CECAM and ESF, the latter coming from the STRUC Psik program.
The format of the workshop was a small and extremely informal meeting (about 20 participants), in the approximate ratio theory/experiment=1/1. All speakers were given a one hour slot, and all participants made extended presentations of their research. Long discussions of points of interest were a typical feature of each presentation. There was also ample time for even less formal discussions during the breaks. An interesting (and unusual) feature of the workshop was the publication of a book of extended abstracts before the workshop. This really helped participants to get up to speed with everyone's recent research.
Huge progress has been made in the last few years with respect to the modelling of structures and properties of complex inorganic structures. Examples that were discussed during the workshop include the calculation of phonon dispersion curves, NMR chemical shifts, compression mechanisms, solid state and surface reactions, optical properties, etc. Similarly, significant progress has been made in the experimental determination of structures and their properties, for example, by using synchrotron radiation from a 3rd generation source in combination with position-sensitive detectors and modern diamond anvil cells for very high pressure studies. The main motivation for the workshop was to assess the current state-ofthe-art since we need to understand the capabilities and limitations of both theoretical and experimental approaches. This is an important consideration when interpreting experimental results, validating a theory by comparing with experimental data, or predicting outcomes of experiments that are difficult to perform. We therefore hope that the contributions to the workshop advanced the mutual understanding of theoreticians, code developers, users of quantum-mechanical programs, and experimentalists, and also provided an outlook of progress to be expected in the next few years.
The organisation of the workshop was an activity of the 'Computational crystallography' group of the German Crystallographic Society, DGK. The proceedings of the meeting are already available in the series 'Berichte aus Arbeitskreisen' published by the DGK (vol. 10; contact Prof. Dr. H. Kfippers, Mineralogisches Institut, Univ. Kiel, D-24098 Kiel, Germany). We are all enormously grateful to Karsten Knorr, one of the organisers of the meeting, for all the efforts that made this publication possible, and to the DGK for an amazingly speedy response.
The meeting's web page:
http://www.min.uni-kiel.de/kristallographie/cecam2000/
gives the information about the workshop program and a list of participants. It is gratifying to note that even after a number of last minute cancellations the meeting had a truly international and interdisciplinary character.
The organisers would like to thank again the host organisation, CECAM, in particular Michel Mareschal and Emmanuelle Crespeau for their competent, efficient, and friendly help. I think it is fair to say that the meeting was successful and enjoyable for participants. A brief description of the contributions follows.
Various aspects of lattice dynamics studies were discussed by Markus Braden (KFZ, Germany and LLB, France) and Stefano de Gironcoli (SISSA, Italy). Markus Braden described recent advances in lattice dynamics measurements at high pressure. For a long time the maximum attainable pressure for neutron scattering experiments was 2-3 GPa, the main limitation being the sample volume needed for reliable phonon measurements. Recent advances in spectrometer and high-pressure cell design allow the limit to be pushed to 10-12 GPa. This opens the way for the search for soft phonon modes in pressure-induced phase transitions, to measure mode Gruneisen parameters (and thus infer a number of thermodynamical properties), and to study pressure dependence of magnetic excitations. Particular results that were presented include the phonon dispersion of Ge up to 10 GPa, phonon and magnon spectra of iron up to 10 GPa (the first ever study of any magnetic excitation under high pressure), etc. It is rather reassuring that existing ab initio predictions of the mode Gruneisen parameter in germanium agree extremely well with the latest reported measurements.
Stefano de Gironcoli presented a review of the developments in the area of density functional perturbation theory followed by an impressive results on geophysical applications with the main emphasis on thermoelasticity of minerals (MgO and perovskite MgSiO3). Ab initio study of elastic constants at high temperature and pressure is now sufficiently accurate that the results of ab initio calculations can be reliably used to make conclusions on applicability of various phenomenological models of the Earth's core, for example.
A number of contributors addressed issued related to electronic transitions and optical properties. Eugene Krasovski (Kiel, Germany) gave an overview of the all-electron methods used to calculate optical properties of crystals within DFT. The experimental viewpoint was presented by Michael Andrut (Vienna, Austria) who gave an excellent introduction to state-of-the-art in infrared and optical absorption spectroscopy of minerals. Michael also described in detail the status of the semiempirical crystal field theory in geosciences. Petra Becker (Cologne, Germany) presented a field of materials for nonlinear optics. Attempts to rationally design new NLO materials are still quite rare, and modelling can help to advance the case of devising a new material that satisfies a number of imposed constraints and has desired nonlinear optical properties. Petra's examples of the recent work on a variety of promising materials (e.g., potassium rare-earth nitrates, bismuth triborate, potassium niobium triborate, alkaline-metal niobium borates) showed just how difficult the quest for an industrially-acceptable NLO material can be.
A large body of discussion was centered around a relatively new field of NMR chemical shifts as an analytical tool in solid state studies. Ian Farnan (Cambridge, UK) introduced the experimental field using 17O studies of silicates as an example. One particular area of interest here is the application of NMR to study element-specific dynamics on a diffusive time scale. These temperature-dependent spectra can become a very good test of molecular dynamics simulations in oxides.
Theoretical developments in the field of NMR calculations were presented in two separate talks by Francesco Mauri (Paris, France) and Chris Pickard (Cambridge, UK). Francesco Mauri introduced the method, which is based on density functional perturbation theory, and presented a number of new results relevant to studies of silicates. The most recent work on disorder in amorphous silica gives a hold on such fine structural details as Si-O-Si angular distribution based on the existing experimental NMR data. Chris Pickard continued the subject by investigating at greater depth the approximations that are made in the theoretical studies, and also by making connections with the NMR methods commonly used in quantum chemistry when studying nonperiodic systems. His set of examples included a number of medium-sized molecular complexes where NMR information is crucial for determining, for example, molecular conformations of porphyrins.
A session on high pressure studies was opened by Michael Hanfland (Grenoble, France) who presented a number of recent high-pressure studies carried out at the European Synchrotron Radiation Facility (ESRF). His examples included the study of phase transitions in solid molecular nitrogen where orientational ordering accompanies the structural changes; pairing of lithium atoms in metallic lithium under compression; and the structural properties of LaFeO3 under pressure. The theoretical part of the high pressure discussions was presented by Bjoern Winkler (Kiel, Germany) in his opening notes and later by Victor Milman (Cambridge, UK). Bjoern Winkler presented the latest results of the DFT study of a mixed valence compound, CsAuCl3, under pressure. DFT calculations for this system reproduce with great accuracy the 'signature' feature of the structure - the change from Au(I)Au(III) electronic configuration to Au(II) which is seen as a creation of a cubic structure with no charge ordering. This finding is encouraging for DFT practitioners in view of recent comments on the necessity of SIC corrections to reproduce mixed-valence compounds correctly. It appears that a straightforward DFT application is sufficiently accurate to describe the structural properties. Further examples of compression studies given by Victor Milman included a recent systematic study of the compressibility of cubic garnets and the study of the phase stability and compression of klockmannite, CuSe. The conclusion is that the DFT results are exceptionally reliable for a large number of compounds and that the bulk modulus is reproduced with an accuracy which is typically better than 10%.
A large section of the workshop was broadly classified as "Structure and Properties". Michele Catti (Milan, Italy) presented recent results on LCAO-HF calculations for silica polymorphs as compared to numerous DFT studies. The main practical conclusion of the long discussion that followed was that the need for analytically calculated gradients in the CRYSTAL code is very strong. Another direction of the discussion was towards clarifying the relative accuracy of LDA and GGA approaches in oxides and related systems. The current situation where GGA is more accurate for some systems and LDA for others, makes systematic studies very difficult.
Julian Gale (London, UK) presented a complete study of crystallisation problems relevant to the aluminium production process. This gave a very clear picture of a complex computational experiment that involves an ab initio study of a number of solid state phases of AI(OH)3, and also a study of a number of molecular reactions in solvents, followed by an ab initio MD simulation of a Bayer liquor.
Marek Hytha (Kiel, Germany) presented a rarely used computational method that involves adaptive selfconsistent pseudopotentials. The basic theory is very straightforward; the only difference with respect to standard pseudopotential DFT implementations is that the pseudopotentials are recalculated with the boundary conditions that are appropriate for a given crystal rather than with the free atom boundary conditions. The resultant scheme preserves the advantages of the pseudopotential method, while giving access to a number of properties like core-level shifts that are not easily available in the standard pseudopotential approaches. The reliability of the scheme was illustrated by a number of examples including layered compounds and silicon (100) surface.
Nicolas Lorente (Toulouse, France) presented theoretical results that shed light on the recent experimental findings from inelastic electron tunneling spectroscopy of molecular adsorbates on metallic surfaces. Ab initio calculations followed by symmetry analysis allow one to interpret experimental results in terms of the vibrational properties of adsorbed species.
A very welcome contribution from Jurgen Schreuer (Zurich, Switzerland) gave a thorough introduction to the world of experimental measurement of elastic constants. A clear and logical description of various techniques was extremely helpful to anyone interested in evaluating the reliability of published experimental data. A subsequent discussion of the recent experimental work on temperature dependence of elastic constants near the phase transition was very interesting.
Karsten Knorr presented a scheme for reconstruction and interpretation of density distributions. It is based on optimal geometric embedding of the fragments into density distributions resulting from maximum entropy reconstructions. A number of examples showed convincingly that a rich variety of structural information can thus be extracted from diffraction data. One of the most impressive cases involves a qualitative representation of a dynamic disorder in the system.
A discussion of various aspects of structure and electronic properties of titanium dichalcogenides was given by Andrei Postnikov (Duisburg, Germany). FLAPW calculations were used to study perfect and doped structures and to relate changes in crystal and electronic structures with the experimental data on transport properties.
Georg Thimm (Singapore) caused a lot of interest in the audience by his talk on topology of crystal structures. The bottom line of the method is that one can generate in a systematic way full sets of all possible structures with given topological constraints. This approach is increasingly used in crystallography for a number of reasons. Firstly, recent experimental findings of various new forms of well-known materials (e.g., carbon) suggest that even more polymorphs might be synthesised, and it is challenging to be able to predict where to look for them. Secondly, computational methods are now sufficiently advanced to be able to process the output of the graph theory methods and end up only with physically viable structures.
The thermodynamics of disorder in minerals was presented by Michele Warren (Manchester, UK). A variety of methods were used, ranging from ab initio simulations on 20-100 atoms to Monte Carlo studies of tens of thousands of cations. The conclusion is that the combination of techniques of different levels of sophistication is capable of describing order-disorder transitions quite accurately.
Bjorn Winkler, Karsten Knorr, Chris Pickard, Victor Milman (November 2000)
More details may be found in newsletter 42 from page 27