Organisers

• Mike Gillan (University College London)
• Alex Shluger (University College London)
• Angelos Michaelides (University College London)

Supports

 CECAM

 Psi-k

 EPSRC Materials Modelling Consortium

 COST - MolSimu

Description

Summary:
The past three years have seen exciting new developments in the calculation of the energetics and spectroscopy of condensed matter using wave-function-based methods, rather than density-functional theory (DFT). These developments are coming from several directions: First, there are vigorous efforts to apply methods that evolved in the condensed-matter community (e.g. plane-wave basis sets) to quantum chemistry techniques (e.g. the MP2 and coupled-cluster approximations); second, major improvements in the scaling of well-established quantum chemistry methods with number of atoms are making it possible to apply these methods to condensed-matter systems; and third, there is major progress in embedding techniques, which allow high-level quantum chemistry or DFT techniques to be embedded in techniques of lower accuracy. The time is now ripe for a discussion workshop that will bring together researchers from both the condensed-matter physics and quantum chemistry communities, to exchange ideas and to explore directions for future developments. We propose a four-day workshop to be held at CECAM in 2008, and we seek joint CECAM/Psi-k funding for this workshop.

Background to proposal:
Since its primitive beginnings 50 years ago, the atomic-scale computer simulation of materials has undergone an extraordinary evolution, in which CECAM has played an important part. In the early days, the simulations were based on simple empirical
models of the interactions between atoms, but these already sufficed to give a reasonable description of some materials, and to calculate a wide range of properties, including structure and diffusion in liquids and solids, static and dynamics surface and defect properties in solids, etc. A major revolution came in 1985, when the Car-Parrinello paper showed how the formulation of quantum mechanics known as density-functional theory (DFT) could be used to model materials as collections of nuclei and electrons, so that the making and breaking of chemical bonds could be modelled in dynamic simulations for the first time. At that time, DFT was associated mainly with the condensed-matter physics community, but it is now widely used by quantum chemists. Over the past 20 years, DFT has gained a dominant position in materials modelling, for several reasons: it
achieves good enough accuracy to identify chemical trends in a very wide range of materials; its computer requirements scale fairly mildly with number of atoms, so that large complex systems can be treated; it is straightforward to achieve basis-set convergence; and atomic forces can be calculated at almost no extra cost, so that high-temperature dynamical and thermodynamic properties can be calculated by molecular dynamics. In particular, DFT statistical mechanics for both bulk materials and for surface
processes is becoming well established.

At the same time, there is a large and important community engaged in tackling materials problems using techniques traditionally associated with the quantum chemistry community. It has been possible for many years to make calculations on large complex
systems using the Hartree-Fock approximation, and with "hybrid" functionals, which combine the DFT and Hartree-Fock theories. These approaches have, until recently, employed Gaussian basis sets, though that situation is now rapidly changing. Furthermore, there is a long-standing and successful effort to calculate the
properties of bulk materials using the so-called "incremental" approach, with high-level wave-function-based techniques, including Moller-Plesset-2 (MP2) and the coupled-cluster hierarchy. A further important approach is the application of quantum-chemistry
techniques to embedded clusters, which in some cases allows the accurate calculation of excited states in the bulk or at surfaces.

All the foregoing developments are well known in their respective communities. But in the past two or three years a new and important current of thought has begun to emerge, and there is a renewed impetus to build bridges between the communities. On the one hand, increasing numbers of DFT practitioners are acknowledging the inadequacies of DFT. (A famous example is the wrong prediction of adsorption sites of molecules on surfaces, and the poor prediction of adsorption energies, but there are many, many other
examples.) This is stimulating some DFT groups to use quantum chemistry to benchmark or correct DFT calculations, and there are examples of DFT and quantum-chemistry research groups teaming up to do this. Other research groups are working to bring key
advantages of the DFT approach, for example automatic basis-set convergence using plane-wave basis sets, into high-level quantum chemistry techniques. On the other hand, there is a drive from the quantum-chemistry side, to exploit recent dramatic improvements in the scaling of wave-function-based methods (so called "local-MP2",
or "local-coupled-cluster") to large systems, including condensed matter. Furthermore, recent reports of wave-function based methods used for molecular dynamics simulations on condensed matter make it realistic to envisage that ways may soon be found of doing most of the things that DFT can do (e.g. DFT statistical mechanics on large complex
systems), but using wave-function based quantum-chemistry techniques that will deliver much better accuracy for many systems.

In our workshop, we want to seize this exciting opportunity to bring together researchers from the DFT and quantum-chemistry communities (or researchers belonging to both), who are actively engaged now in developing and promoting these newly emerging ambitions. We note that the three organisers cover together the three-fold theme of DFT, quantum chemistry and statistical mechanics, and the research groups of all three are working in the directions we have described. The general aims of the workshop are to examine and compare the different strategies that are being developed for using quantum chemistry techniques to study condensed matter, to consider future possibilities that are now emerging, and hopefully to stimulate the formation of new collaborations.
We note that a workshop having something in common with what we propose was held in September 2007 at the Max-Planck-Institut fuer Physik komplexer Systeme in Dresden. This was entitled "Local correlation methods: From molecules to crystals", and was organised by Birkenheuer, Schuetz, Pisani and Paulus. However, the workshop proposed here will, we believe, be broader, since it will place a strong emphasis on drawing together researchers from different backgrounds. For this reason, there will be a major difference in the topics presented, and we expect there to be only a small overlap of the lists of participants of the two workshops.

Scientific Objectives

Objectives of workshop:
The overall aim of the workshop is to explore in some detail the directions that are now being pursued by different research groups to apply wave-function-based techniques to study the energetics, statistical mechanics and spectroscopy of condensed matter. As part of this, the organisers regard it as very important to bring together researchers from different backgrounds, and particularly from the backgrounds of materials and molecular chemistry, and condensed-matter physics. In addition to helping members of the different communities to exchange ideas with each, we envisage that the workshop may well be an important opportunity to initiate new collaborations between research groups.

In order to provide a clear structure, the workshop will be organised around a number of important questions, some of which are as follows:

1. What are the most effective strategies for achieving systematic basis-set convergence for condensed matter within size-extensive quantum chemistry techniques such as the Hartree-Fock approximation, the 2nd-order Moller-Plesset theory, and the coupled-cluster hierarchy, and what are the relative merits of the strategies?

2. Given the recent major improvements in the scaling of correlated calculations (MP2 and coupled-cluster) with number of atoms, what are the new opportunities for calculations on condensed-matter systems made possible by these improvements? What is the importance of recent progress in developing linear-scaling Hartree-Fock techniques?

3. What is the accuracy that can now be achieved with wave-function-based techniques in the calculation of fundamental properties of bulk crystals (cohesive energy, equilibrium lattice parameters, elastic and dielectric constants, vibrational frequencies...)?
How do things stand with the calculation of the properties of material surfaces (surface formation energy, relaxed surface structure), and the energetics of molecular adsorption on surfaces? What are the limitations of wave-function-based methods, and where are these
approaches unlikely to help? How can wave-function-based and DFT methods be best combined in a complementary fashion?

4. What is the state of the art in the development of embedding techniques, in which high-level quantum chemistry methods are applied to clusters, with the surrounding material treated at a lower level? (This question includes, of course, the well-known QMMM methods, but we intend that there will be an emphasis on recent progress, for example the embedding of wave-function based correlated methods in a DFT description.)

5. What new ideas are emerging for the calculation of electronic excited states, both in bulk materials and at their surfaces?

6. Using either embedding methods or periodic boundary conditions, what accuracy can now be achieved for the calculation of energy barriers associated with reactions at surfaces or with diffusion processes in bulk materials?

7. What directions are emerging for the use of wave-function-based methods to study materials in thermal equilibrium, and in particular to calculate thermodynamic free energies, for example the free energies of reaction barriers or the free energies of adsorption of molecules on surfaces?

8. What are the computational needs and bottlenecks of currently emerging methods for applying quantum chemistry to condensed matter? Are there prospects for exploiting peta-scale computers for this purpose? Will the current supercomputing trend towards larger numbers of processors, each having less memory, be a help or a hindrance in the application of quantum chemistry techniques to condensed matter?

It goes without saying that other important questions can also be asked, and the organisers will be open to suggestions from participants at any time.

The above issues are all methodological, but we strongly believe that progress can only be demonstrated by the ability to address practical problems. Participants will therefore be encouraged, so far as possible, to support their ideas with the results of their practical calculations.

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