Relativistic Effects in Heavy-Element Chemistry and Physics

Description

In August 1991, seven scientists from seven European Countries, working in the fieldof relativistic electronic structure theory, met in Strasbourg as guests of Dr ManfredMahnig, who was at this time a representative of the European Science Foundation.They gathered in order to discuss the state of the art in relativistic electronic structuretheory of atoms and molecules and to explore the possibilities of promoting the field ata European scale. The initiative for this meeting can be traced back to correspondencebetween Dr Mahnig and Professor Pekka Pyykko, which can therefore be taken asthe germ of the various REHE programmes promoting the field of relativistic effectsin heavy-element chemistry and physics. The first was the REHE programme of theEuropean Science Foundation, which was current in the years 1993–1998 and provedextremely effective by providing the opportunity for many scientists to take advantageof short visits and also longer visits lasting up to several weeks to the most activelaboratories in the field all over Europe. In addition, a series of Euroconferences andworkshops ensured a rapid exchange of ideas. The newsletter of the program providedrapid exchange of information between the participating groups. The programme wasdirected by a steering committee comprising E. J. Baerends (Amsterdam), J. P. Daudey(Toulouse), K. Faegri (Oslo), I. P. Grant (Oxford), B. Hess (Bonn, Vice-Chairman),J. Karwowski (Torun), P. Pyykko (Helsinki, Chairman), K. Schwarz (Vienna) and A.Sgamellotti (Perugia). The success of this programme is documented by an impressivelist of papers which received funding by REHE.The European REHE programme fostered rapid development of the area, and soon aCollaborative Research Program ('Schwerpunkt') programme of the German ScienceFoundation (DFG) was approved, which in the years 1994–2000 provided funding forabout 30 research groups, mostly in the form of positions for graduate students and, insome cases, postdoctoral researchers. This funding was granted based on applicationsof individual research groups, which were reviewed in a collaborative context everysecond year by an international committee of referees. The collaborative aspect wasstrengthened by reports by the groups at yearly meetings, which soon led to newcollaborations and exchanges of ideas across the participating groups.The topic of the REHE programmes was relativistic effects in heavy-element chemistry,but what are relativistic effects? In principle, the answer is easy. A relativisticeffect is any phenomenon which can be traced back to the fact that the velocity oflight is a universal, finite constant in all frames of reference, even those moving withsome unchanging velocity with respect to each other. Thus, a relativistic effect relieson a comparison with a fictitious world where the velocity of light is infinite and a'nonrelativistic' description applies.There is still discussion, in particular, in experimentally oriented papers, aboutwhether relativistic effects 'really exist' and are measurable, or if they are an artefactof a 'wrong theory', namely, the nonrelativistic one, and cannot be measured becausein reality there are no nonrelativistic atoms. However, since the notion of relativisticeffects is well defined, I claim that they can even be measured in favourable casesdirectly from the behaviour of a simple function of the atomic number Z. Consider thebinding energies of the 1s electron of hydrogen-like atoms, which are well accessibleto measurement. Obviously, this quantity depends on Z, and we attribute the dependenceon Z beyond second order to relativity, since nonrelativistic theory predicts thatthere are no nonvanishing Taylor coefficients beyond second order. The relativisticeffect therefore can in this particular case be measured as the deviation of E as afunction of Z from parabolic behaviour, a very simple prescription indeed.It is remarkable that the Dirac theory of the relativistic electron perfectly describesthis deviation, and the difference to the reference (the nonrelativistic value) is unusuallywell defined by the limit of a single parameter (the velocity of light) at infinity.The special difficulty encountered in 'measuring' relativistic effects is that relativisticquantum mechanics is by no means a standard part of a chemist's education, andtherefore the theory for interpreting a measurement is often not readily at hand. Still,a great many of the properties of chemical substances and materials, in particular,'trends' across the periodic system of elements, can be understood in terms of relativisticeffects without having to consider the details of the theory.Needless to say, many-electron atoms and molecules are much more complicatedthan one-electron atoms, and the realization of the nonrelativistic limit is not easilyaccomplished in these cases because of the approximations needed for the descriptionof a complicated many-particle system. However, the signature of relativistic effects(see, for example, Chapter 3 in this book) enables us to identify these effects even withoutcalculation from experimental observation. Two mainly experimentally orientedchapters will report astounding examples of relativistic phenomenology, interpretedby means of the methods of relativistic electronic structure theory. These methods forthe theoretical treatment of relativistic effects in many-electron atoms and moleculesare the subject of most of the chapters in the present volume, and with the help of thistheory relativistic effects can be characterized with high precision.The present book serves a twofold purpose. On one hand, the book was designedto serve as a final report on the work done in the Collaborative Research Programme('Schwerpunkt') of the German Science Foundation on Relativistic Effect in Heavy-Element Chemistry and Physics. I apologize that for that reason it is certainly biasedtowards the work of the groups who had participated in the last period of this programme,and as a consequence our account will certainly be found to have missedimportant contributions to the field. On the other hand, to some extent it should alsogive an account of the worldwide progress made in the last decade as far as the methodologyof calculating relativistic effects in heavy-element chemistry and physics andtheir interpretation are concerned. Thus, we have made some effort to review the workdone in Europe, in particular, in the framework of the REHE programme, and all overthe world in the field of relativistic electronic structure calculations.On behalf of the participants of the 'German REHE Schwerpunkt', I thank theGerman Science Foundation for their generous and highly effective funding. Theprofessional guidance through the technical aspects of the programme by Dr Carnell,Dr Kuchta and Dr Mahnig, who were the officers of the DFG in charge of the'Schwerpunkt', was instrumental to its success. Since Dr Mahnig also guided theearly stages of the 'European REHE' at the European Science Foundation, it is fairto say that without his help and initiative in the early 1990s the REHE programmeswould not exist, nor would the present book. An important instrument in the design ofa 'Schwerpunkt' is the refereeing process, carried out by an international committeeof experts, all of them with illustrious reputations in the field. Their presence at themeetings was highly appreciated and without doubt provided many stimuli whichfound their way into the work of the groups refereed by them. I express my sincerestgratitude for the time and the work they have devoted to the programme.Finally, I also thank the participants of the last term of this programme for supplyinga large amount of the material which has been used by the authors of the seven chaptersof this book in order to accomplish the task of providing a report on six years ofresearch in a fascinating area of modern science.