Preface: phys. stat. sol. (b) 243/5
Authors: Artacho Emilio , Beck Thomas L. , Hernández Eduardo 
Published: March 2006

Between 20 and 24 June 2005 the Centre Européen de Calcul Atomique et Moléculaire – or CECAM, as it is more widely known – hosted a workshop entitled State‐of‐the‐art, developments and perspectives of real‐space electronic structure methods in condensed‐matter and chemical physics, organized with the support of CECAM itself and the Ψk network. The workshop was attended by some forty participants coming from fifteen countries, and about thirty presentations were given. The workshop provided a lively forum for the discussion of recent methodological developments in electronic structure calculations, ranging from linear‐scaling methods, mesh techniques, time‐dependent density functional methods, and a long etcetera, which had been our ultimate objective when undertaking its organization.

The first‐principles simulation of solids, liquids and complex matter in general has jumped in the last few years from the relatively confined niches in condensed matter and materials physics and in quantum chemistry, to cover most of the sciences, including nano, bio, geo, environmental sciences and engineering. This effect has been propitiated by the ability of simulation techniques to deal with an ever larger degree of complexity. Although this is partially to be attributed to the steady increase in computer power, the main factor behind this change has been the coming of age of the main theoretical framework for most of the simulations performed today, together with an extremely active development of the basic algorithms for its computer implementation. It is this latter aspect that is the topic of this special issue of physica status solidi.

There is a relentless effort in the scientific community seeking to achieve not only higher accuracy, but also more efficient, cost‐effective and if possible simpler computational methods in electronic structure calculations [1]. From the early 1990s onwards there has been a keen interest in the computational condensed matter and chemical physics communities in methods that had the potential to overcome the unfavourable scaling of the computational cost with the system size, implicit in the momentum–space formalism familiar to solid‐state physicists and the quantum chemistry approaches more common in chemical physics and physical chemistry. This interest was sparkled by the famous paper in which Weitao Yang [2] introduced the Divide and Conquer method. Soon afterwards several practical schemes aiming to achieve linear‐scaling calculations, by exploiting what Walter Kohn called most aptly the near‐sightedness of quantum mechanics [3], were proposed and explored (for a review on linear‐scaling methods, see [4]). This search for novel, more efficient and better scaling algorithms proved to be fruitful in more than one way. Not only was it the start of several packages which are well‐known today (such as Siesta, Conquest, etc.), but it also leads to new ways of representing electronic states and orbitals, such as grids [5, 6], wavelets [7], finite elements, etc. Also, the drive to exploit near‐sightedness attracted computational solid state physicists to the type of atomic‐like basis functions traditionally used in the quantum chemistry community. At the same time computational chemists learnt about plane waves and density functional theory, and thus a fruitful dialogue was started between two communities that hitherto had not had much contact.

Another interesting development that has begun to take place over the last decade or so is the convergence of several branches of science, notably physics, chemistry and biology, at the nanoscale. Experimentalists in all these different fields are now performing highly sophisticated measurements on systems of nanometer size, the kind of systems that us theoreticians can address with our computational methods, and this convergence of experiment and theory at this scale has also been very fruitful, particularly in the fields of electronic transport and STM image simulation. It is now quite common to find papers at the cutting edge of nanoscience and nanotechnology co‐authored by experimentalists and theorists, and it can only be expected that this fruitful interplay between theory and experiment will increase in the future.

It was considerations such as these that moved us to propose to CECAM and Ψk the celebration of a workshop devoted to the discussion of recent developments in electronic structure techniques, a proposal that was enthusiastically received, not just by CECAM and Ψk, but also by our invited speakers and participants. Interest in novel electronic structure methods is now as high as ever, and we are therefore very happy that physica status solidi has given us the opportunity to devote a special issue to the topics covered in the workshop. This special issue of physica status solidi gathers invited contributions from several attendants to the workshop, contributions that are representative of the range of topics and issues discussed then, including progress in linear scaling methods, electronic transport, simulation of STM images, time‐dependent DFT methods, etc. It rests for us to thank all the contributors to this special issue for their efforts, CECAM and Ψk for funding the workshop, physica status solidi for agreeing to devote this special issue to the workshop, and last but not least Emmanuelle and Emilie, the CECAM secretaries, for their invaluable practical help in putting this workshop together.