Session F44: Van Der Waals Interactions in Molecules, Materials, and Complex Environments I

Quantum-mechanical van der Waals Interactions from Atoms to Asteroids

Noncovalent van der Waals (vdW) interactions arise from quantum-mechanical electronic and atomic fluctuations and they are ubiquitous in essentially all molecules and materials [RMP 88, 045003; Chem. Rev. 117, 4714; Science 351, 1171]. Intergrain vdW interactions have even been hypothesized to explain cohesion in kilometer-sized rubble pile asteroids in space [Nature 512, 174]. I will review the substantial progress in our understanding of everpresent quantum vdW forces at the atomic, nano, and mesoscopic scales achieved during the last two decades. Then I will turn to big gaps in our knowledge where intensive research is required, including (i) the unification of vdW methods with semilocal density functionals, (ii) the scaling of vdW forces with topology, dimensionality, size, and electronic properties of materials, (iii) the interplay between atomic vibrations and electronic fluctuations in the vdW forces, (iv) the delicate transition between vdW and Casimir physics in complex systems.   

Dispersion-corrected MP2 for improved descriptions of inter- and intramolecular interactions in organic molecular crystals

Reliably modeling polymorphic molecular crystals requires a careful balance between intra- and intermolecular interactions. Density-functional theory models have made tremendous progress in modeling such interactions, but a number of failures for conformational polymorphs and other systems can be found in the literature. A dispersion-corrected MP2 model, MP2D, will be presented which provides more accurate descriptions of non-covalent interactions in these challenging polymorphic crystals. The MP2D dispersion correction is expressed in terms of atom-centered dispersion coefficients which are computed via Grimme's D3 scheme. This approach has the benefits that the dispersion correction can be computed for any organic system with negligible computational cost, it can be applied to both intra- and intermolecular interactions, and analytical gradients can be implemented trivially. The performance of MP2D for both benchmark data sets and in challenging polymorphich molecular crystals will be presented.

  

Van der Waals Attraction and Pauli Repulsion: Learning New Tricks from an Old Dog

The structure and stability of molecular systems bonded by van der Waals (vdW) interactions are governed by the interplay between dispersion attraction and Pauli repulsion [Hermann et al., Chem. Rev. 117, 4714 (2017)]. Arising due to different physical origins, these forces do not seem to have a simple connection. Here, we present a coarse-grained approach for evaluating the exchange energy based on the multipole expansion of the Coulomb interaction. This allows us to reveal an unexpected compensation between attractive dispersion and repulsive exchange forces for closed-shell dimers at equilibrium distance, valid for each term of the multipole expansion. This effect explains the surprisingly simple relationship between the vdW radius and atomic polarizabilities discovered recently [Fedorov et al., PRL 121, 183401 (2018)]. The obtained recipe to build coarse-grained models for the exchange-repulsive forces in vdW-bonded systems could be used to develop next-generation quantum force fields. Therefore, our findings hint on a hidden symmetry between exchange and correlation interactions and suggest a surprising connection between electronic and geometric properties of atoms.   

Characterization of van der Waals Interactions with Energy Decomposition Analysis

Van der Waals (vdW) interactions are ubiquitous in nature, and represent important contributions to the structure, stability, and function of many chemical systems. The absolutely localized molecular orbital energy decomposition analysis method (ALMO-EDA) allows for the characterization and quantification of intermolecular interactions such as London dispersion, the predominant attractive vdW component which is due to correlated fluctuations of the electron densities between molecules, and Pauli repulsion, the predominant repulsive vdW component which is a consequence of the antisymmetry of fermionic wave functions. Here we discuss our work studying vdW interactions calculated with ALMO-EDA in small and large molecular systems, and our findings regarding the role of nonlocal terms in approximate exchange-correlation density functionals in predicting vdW interaction energies, as well as many-body cooperativity in vdW interactions.

J. Chem. Theory Comput. 2019, 15, 2983-2995   

Introducing vdW-DF3 — an accurate van der Waals Density Functional

Recently, we identified the shape of the switching function in the original van der Waals density functional vdW-DF as the key to control the relative contributions of dispersion interactions at different separations (PRB 99, 195418, 2019). Building on this development, we introduce a reformulation that results in significantly higher accuracy for several different classes of systems, in particular for separations beyond the binding distance. This is achieved through tuning the switching function (and thus the non-local correlation) as well as the (semi)local exchange to a set of molecular dimers. Our new formulation provides better binding energies at the equilibrium distance and binding curves of molecular dimers, more accurate interlayer binding energies and lattice constants for layered systems, and improved binding energies for adsorption systems such as small-molecule-adsorption in MOFs and benzene on coinage metals.   

Experimental determination of van der Waals forces between two-dimensional materials in air and water

Two-dimensional (2D) materials have rather diverse properties and are therefore of high interest for applications in material science, physics, chemistry, and biology. Since many of these applications rely on interfacial stacking, a fundamental understanding of the interactions between 2D materials is of paramount importance. In this work, we describe a strategy which enables rapid and high-throughput determination of the forces between 2D materials (both in air and in liquids) using atomic force spectroscopy. While our experiments demonstrate strong adhesive forces between two graphene sheets with binding energies close to theoretical values, the scaling of the van der Waals (vdW) force with the separation distance differs quite substantially from theoretical predictions. In addition, our experiments in purified water demonstrate that the water confined between two hydrophobic surfaces leads to repulsive interactions, with increasing ionic strength reverting this behavior. These experiments challenge our current understanding of the vdW force and provide much needed experimental benchmarks for the further development of theoretical vdW methodologies.
  

Influence of Pore Size on the van der Waals Interaction in Two-Dimensional Molecules and Materials

Despite the importance of porous two-dimensional (2D) molecules and materials in advanced technological applications, the question of how the void space in these systems affects the van der Waals (vdW) scaling landscape has been largely unanswered. In this work [1], we present a series of analytical and numerical models demonstrating that the mere presence of a pore leads to markedly different vdW scaling across non-asymptotic distances, with certain relative pore sizes yielding effective power laws ranging from simple monotonic decay to the formation of minima, extended plateaus, and even maxima. These models are in remarkable agreement with first-principles approaches for the 2D building blocks of covalent organic frameworks (COFs), and reveal that COF macrocycle dimers and periodic bilayers exhibit unique vdW scaling behavior that is quite distinct from their nonporous analogs. These findings extend across a range of distances relevant to the nanoscale, and represent an unexplored avenue towards governing the self-assembly of complex nanostructures from porous 2D molecules and materials.

[1] Y. Yang, K. U. Lao, and R. A. DiStasio Jr, Phys. Rev. Lett. 122, 026001 (2019).   

Ultra Long-Range Interactions in the Delamination of Atomically-Thin Layers from Substrates

Anomalous proximity effects have been experimentally observed in systems ranging from proteins, bacteria, and gecko feet suspended over semiconductor surfaces to interfaces between graphene and different substrate materials (Si, SiO2, Cu). In the latter case, long-range forces are evidenced by measurements of a non-vanishing stress that extends up to micrometer separations between graphene and the substrate. State-of-the-art models to describe adhesive properties are unable to explain these experimental observations, instead underestimating the measured distance range by 2-3 orders of magnitude. Here we develop an analytical and numerical variational approach that combines continuum mechanics and elasticity with quantum many-body treatment of van der Waals dispersion interactions between two extended objects. A full relaxation of the coupled adsorbate/substrate geometry as a function of separation leads us to conclude that wavelike atomic deformation is responsible for the observed ultra long-range stress in delamination of graphene from various substrates. Remarkably, the observed long-range proximity effect seems to be a general phenomenon for thin membranes and its correct theoretical description requires a direct coupling between quantum and continuum mechanics.   

Hidden by graphene: effective screening of interface van der Waals interactions via monolayer coating

Recent atomic force microscopy experiments [ACS Nano 2014, 8, 12410–12417] conducted on graphene (G)-coated SiO₂ demonstrated that monolayer G can effectively screen dispersion van der Waals (vdW) interactions deriving from the underlying substrate. This G vdW opacity has far reaching implications, encompassing stabilization of multilayer heterostructures, micromechanical phenomena and heterogeneous catalysis. By quantum many-body analysis and ab-initio Density Functional Theory, here we provide theoretical rationalization of the observed G vdW screening on weakly interacting substrates: despite single atom thickness, the strong non-locality of G density response ensures large compensation between standard attractive vdW terms and many-body repulsive contributions, enabling effective vdW opacity over a broad range of distances. By virtue of combined theoretical/experimental validation, G hence emerges as a promising ultrathin shield for modulation and switching of vdW interactions and electrostatic fields at interfaces and complex nanoscale devices.