Van der Waals (vdW) interactions are ubiquitous in nature, playing a major role in defining the structure, stability, and function for a wide variety of molecules and materials. An accurate first-principles description of vdW interactions is extremely challenging, since the vdW dispersion energy arises from the correlated motion of electrons and, in principle, must be described by many-electron quantum mechanics. Rapid increase in computer power and advances in theoretical models for vdW interactions have allowed to achieve "chemical accuracy" for binding between small organic molecules. However, the lack of accurate and efficient methods for large and complex systems hinders truly quantitative predictions of properties and functions of technologically relevant materials. Typical applications where vdW interactions play an essential role include the design of novel hybrid inorganic/organic interfaces for photovoltaics, energy storage, and sensor devices , understanding the structure and dynamics of drug binding to proteins, as well as the creation of "smart" nanomechanical devices with tunable electronic properties .
Within this context our proposed CECAM Workshop "Towards First-Principles Description of van der Waals Interactions in Complex Materials" wants to be an opportunity to (i) bring the major players up to date in the most recent developments in the field, (ii) identify a set of (complex) systems -- beyond small molecules -- that can be used to benchmark newly developed methods, (iii) discuss the implementation and validation of different vdW methods in electronic structure codes, and (iv) identify future major challenges in the area of modeling vdW interactions.
This workshop finds its motivation in the realization that, while the vdW interactions are constantly found to be significantly more important than anticipated for a broad range of materials, there are still few initiatives on bringing together both the developers and users of vdW methods at the same place and time. We therefore propose a focused workshop on vdW interactions in complex materials, with a strong emphasis on the collaboration between developers and users. This workshop is extremely timely as the different methods for vdW interactions have been or are in the process of being implemented in most major electronic structure software projects.
Density-functional theory (DFT) is currently the method of choice for the modeling of complex materials. During the last decade there has been a surge of interest in developing new methods for the modeling of vdW interactions in DFT [3, 4, 5, 6]. Loosely speaking, three successful (and somewhat different) ways can be identified: (i) Non-local density functionals, capturing the pairwise (two-body) part of the vdW energy [7, 8, 9, 10]; (ii) Interatomic (pairwise) vdW potentials, added to the DFT total energy in a post-processing fashion [11, 12, 13, 14, 15]; (iii) Computationally expensive DFT functionals on the fifth rung of Perdew's Jacob's ladder, which explicitly include Coulomb screening and many-body vdW energy [16, 17, 18, 19, 20].
Despite significant progress in the field of modeling vdW interactions during the last decade, many questions remain unanswered and much development needs to be done before a truly universally applicable (accurate and efficient) method emerges. For example, the employed approximations and the connections between different ways of modeling vdW interactions [(i), (ii), (iii) above] are not always transparent. Furthermore, the domain of applicability of every method is not clearly defined. For example, interatomic vdW potentials are frequently employed for the modeling of hybrid inorganic/organic interfaces [21, 22, 23, 24], neglecting the rather strong Coulomb screening present within inorganic bulk materials. On the other hand, the popular non-local vdW-DF functionals [7, 8] use a purely local approximation for the polarizability, which is not expected to be accurate for molecules. Nevertheless, the interaction energies between organic molecules turn out to be reasonably accurate. Understanding the physical reasons of why these different approaches "work" outside of their expected domain of applicability is important for the development of more robust approximations.