Long-range Van Der Waals Interactions Research Articles

Dipole-dipole-interaction–driven antiblockade of two Rydberg atoms

Resonant laser excitation of multiple Rydberg atoms are prohibited, leading to Rydberg blockade, when the long-range van der Waals interactions are stronger than the laser-atom coupling. Rydberg blockade can be violated, i.e., simultaneous excitation of more than one Rydberg atom, by off-resonant laser excitation, causing an excitation antiblockade. Rydberg antiblockade gives rise to strongly correlated many-body dynamics and spin-orbit coupling and also finds quantum computation applications. Instead of commonly used van der Waals interactions, we investigate antiblockade dynamics of two Rydberg atoms interacting via dipole-dipole exchange interactions. We study typical situations in current Rydberg atom experiments, where different types of dipole-dipole interactions can be achieved by varying Rydberg state couplings. An effective Hamiltonian governing underlying antiblockade dynamics is derived. We illustrate that geometric gates can be realized with the Rydberg antiblockade which is robust against the decay of Rydberg states. Our study may stimulate new experimental and theoretical exploration of quantum optics and strongly interacting many-body dynamics with Rydberg antiblockade driven by dipole-dipole interactions.

Van der Waals interaction as the starting point for an effective field theory

We consider the system of three ${}^4$He atoms to assess whether a pure van der Waals potential can be used as a starting point for an effective field theory to describe three-body processes in ultracold atomic systems. Using a long-range van der Waals interaction in combination with short-distance two-body counterterms, we analyze the dependence of two- and three-body observables on the short-distance regulator that is required due to the singular nature of the van der Waals interaction. We benchmark our approach with results obtained with the realistic ${}^4$He-${}^4$He LM2M2 interaction and find good agreement. We furthermore show that in this effective field theory approach no three-body force is required at leading order and that universal van der Waals physics leads to a universal correlation between three-body observables in the absence of an Efimov three-body parameter.

CrI3 revisited with a many-body ab initio theoretical approach

${\mathrm{CrI}}_{3}$ has recently been shown to exhibit low-dimensional, long-range magnetic ordering from few layers to single layers of ${\mathrm{CrI}}_{3}$. The properties of ${\mathrm{CrI}}_{3}$ bulk and few-layered systems are uniquely defined by a combination of short-range intralayer and long-range interlayer interactions, including strong correlations, exchange, and spin-orbit coupling. Unfortunately, both the long-range van der Waals interactions, which are driven by dynamic, many-body electronic correlations, and the competing strong intralayer correlations, present a formidable challenge for the local or semilocal mean-field approximations employed in workhorse electronic structure approaches like density-functional theory. In this paper we employ a sophisticated many-body approach that can simultaneously describe long- and short-range correlations. We establish that the fixed-node diffusion Monte Carlo (FNDMC) method reproduces the experimental interlayer separation distance of bulk ${\mathrm{CrI}}_{3}$ for the high-temperature monoclinic phase with a reliable prediction of the interlayer binding energy. We subsequently employed the FNDMC results to benchmark the accuracy of several density-functional theory exchange-correlation approximations.

Many-body van der Waals interactions beyond the dipole approximation

Long-ranged van der Waals (vdW) interactions are most often treated via Lennard-Jones approaches based on the combination of two-body and dipolar approximations. While beyond-dipole interactions and many-body contributions were separately addressed, little is known about their combined effect, especially in large molecules and relevant nanoscale systems. Here, we provide a full many-body description of vdW interactions beyond the dipole approximation, efficiently applicable to large-scale systems. Dipole-quadrupole interactions consistently exhibit large magnitude up to nm-scale separations, while many-body effects lead to system-dependent screening effects, which can reduce vdW interactions by a large fraction. Combined many-body and multipolar terms emerge as an essential ingredient for the reliable description of vdW interactions in molecular and nanoscale systems.

Partial Order–Disorder Transformation of Interpenetrated Porous Coordination Polymers

Controlling interpenetration and flexibility behaviors is intriguing and fundamental for porous coordination polymers. We report exceptional interpenetration behaviors involving controllable partia...

Describing adsorption of benzene, thiophene, and xenon on coinage metals by using the Zaremba-Kohn theory-based model.

Semilocal (SL) density functional approximations (DFAs) are widely applied but have limitations due to their inability to incorporate long-range van der Waals (vdW) interaction. Non-local functionals (vdW-DF, VV10, and rVV10) or empirical methods (DFT+D, DFT+vdW, and DFT+MBD) are used with SL-DFAs to account for such missing interaction. The physisorption of a molecule on the surface of the coinage metals (Cu, Ag, and Au) is a typical example of systems where vdW interaction is significant. However, it is difficult to find a general method that reasonably describes both adsorption energy and geometry of even the simple prototypes of cyclic and heterocyclic aromatic molecules such as benzene (C6H6) and thiophene (C4H4S), respectively, with reasonable accuracy. In this work, we present an alternative scheme based on Zaremba-Kohn theory, called DFT+vdW-dZK. We show that unlike other popular methods, DFT+vdW-dZK and particularly SCAN+vdW-dZK give an accurate description of the physisorption of a rare-gas atom (xenon) and two small albeit diverse prototype organic molecules on the (111) surfaces of the coinage metals.

Challenges for machine learning force fields in reproducing potential energy surfaces of flexible molecules.

Dynamics of flexible molecules are often determined by an interplay between local chemical bond fluctuations and conformational changes driven by long-range electrostatics and van der Waals interactions. This interplay between interactions yields complex potential-energy surfaces (PESs) with multiple minima and transition paths between them. In this work, we assess the performance of the state-of-the-art Machine Learning (ML) models, namely, sGDML, SchNet, Gaussian Approximation Potentials/Smooth Overlap of Atomic Positions (GAPs/SOAPs), and Behler-Parrinello neural networks, for reproducing such PESs, while using limited amounts of reference data. As a benchmark, we use the cis to trans thermal relaxation in an azobenzene molecule, where at least three different transition mechanisms should be considered. Although GAP/SOAP, SchNet, and sGDML models can globally achieve a chemical accuracy of 1 kcal mol-1 with fewer than 1000 training points, predictions greatly depend on the ML method used and on the local region of the PES being sampled. Within a given ML method, large differences can be found between predictions of close-to-equilibrium and transition regions, as well as for different transition mechanisms. We identify key challenges that the ML models face mainly due to the intrinsic limitations of commonly used atom-based descriptors. All in all, our results suggest switching from learning the entire PES within a single model to using multiple local models with optimized descriptors, training sets, and architectures for different parts of the complex PES.

Coulomb interactions between dipolar quantum fluctuations in van der Waals bound molecules and materials

Mutual Coulomb interactions between electrons lead to a plethora of interesting physical and chemical effects, especially if those interactions involve many fluctuating electrons over large spatial scales. Here, we identify and study in detail the Coulomb interaction between dipolar quantum fluctuations in the context of van der Waals complexes and materials. Up to now, the interaction arising from the modification of the electron density due to quantum van der Waals interactions was considered to be vanishingly small. We demonstrate that in supramolecular systems and for molecules embedded in nanostructures, such contributions can amount to up to 6 kJ/mol and can even lead to qualitative changes in the long-range van der Waals interaction. Taking into account these broad implications, we advocate for the systematic assessment of so-called Dipole-Correlated Coulomb Singles in large molecular systems and discuss their relevance for explaining several recent puzzling experimental observations of collective behavior in nanostructured materials.

Benchmarking dispersion-corrected DFT methods for the evaluation of materials with anisotropic properties: structural, electronic, dielectric, optical and vibrational analysis of calcite (CaCO3, space group R3[combining macron]c).

Calcite (CaCO3, space group R3[combining macron]c) is a solid phase whose well-known highly anisotropic physical properties can be exploited to compare and calibrate various theoretical simulation methods. In this work, to benchmark different ab initio Density Functional Theory approaches that include for the first time corrections for dispersive forces, a systematic analysis of structural, electronic, dielectric, optical and vibrational properties of calcite is performed. The simulations considered the generalized-gradient approximation functional PBE and the hybrid B3LYP and PBE0, whereas the DFT-D2 and DFT-D3 schemes were adopted to account for the long-range interactions. This study suggests an overall better agreement between the theoretical results obtained with the DFT functionals corrected for the dispersive forces, with a better performance of hybrid functionals over PBE.

Structural relaxation and low-energy properties of twisted bilayer graphene

The structural and electronic properties of twisted bilayer graphene are investigated from first principles and tight binding approach as a function of the twist angle (ranging from the first "magic" angle $\theta=1.08^\circ$ to $\theta=3.89^\circ$, with the former corresponding to the largest unit cell, comprising 11164 carbon atoms). By properly taking into account the long-range van der Waals interaction, we provide the patterns for the atomic displacements (with respect to the ideal twisted bilayer). The out-of-plane relaxation shows an oscillating ("buckling") behavior, very evident for the smallest angles, with the atoms around the AA stacking regions interested by the largest displacements. The out-of-plane displacements are accompanied by a significant in-plane relaxation, showing a vortex-like pattern, where the vorticity (intended as curl of the displacement field) is reverted when moving from the top to the bottom plane and viceversa. Overall, the atomic relaxation results in the shrinking of the AA stacking regions in favor of the more energetically favorable AB/BA stacking domains. The measured flat bands emerging at the first magic angle can be accurately described only if the atomic relaxations are taken into account. Quite importantly, the experimental gaps separating the flat band manifold from the higher and lower energy bands cannot be reproduced if only in-plane or only out-of-plane relaxations are considered. The stability of the relaxed bilayer at the first magic angle is estimated to be of the order of 0.5-0.9 meV per atom (or 7-10 K). Our calculations shed light on the importance of an accurate description of the vdW interaction and of the resulting atomic relaxation to envisage the electronic structure of this really peculiar kind of vdW bilayers.

N-Annulated Perylene Bisimides to Bias the Differentiation of Metastable Supramolecular Assemblies into J- and H-Aggregates.

The unique self-assembling features of N-annulated perylene bisimides (PBIs) 1 and 2 are reported. The stability of the aggregates of diester 1, in which no H-bonding interactions are operative, corroborates the significance of long-range van der Waals and dipole-dipole electrostatic interactions in the construction of stable supramolecular assemblies. The incorporation of amide functional groups within the N-annulated PBI in 2 stimulates pathway differentiation to achieve up to three J-type aggregates and a fourth H-type aggregate depending on the experimental conditions. The results presented demonstrate unprecedented levels of control over synthetic supramolecular self-assembly and the rich differentiation that N-annulated PBIs exhibit, opening the door to new, complex, functional supramolecular materials.

Density functionals combined with van der Waals corrections for graphene adsorbed on layered materials

Standard density functionals like Perdew-Burke-Ernzerhof (PBE) or Strongly Constrained and Appropriately Normed (SCAN) need a correction to account for long-range van der Waals (vdW) interaction. The damped Zaremba-Kohn model (dZK) [J. Tao, H. Tang, A. Patra, P. Bhattarai, and J. P. Perdew, Phys. Rev. B 97, 165403 (2018)] starts from a formula for the vdW interaction of a distant atom with a solid surface, both with known dielectric properties, damps this formula at short range, and then treats an adsorbed molecule or atomic layer as a collection of renormalized atoms. We extend this model to graphene adsorbed on semiconducting layered materials [bulk graphite and hBN, and multilayer transition metal dichalcogenides (TMD)] by including the ${C}_{4}$ asymptotic term and multiple electrostatic image effects due to the two surfaces of the substrate slabs in the vdW calculations. The resulting SCAN-vdW-dZK and PBE-vdW-dZK give approximately the same results for the systems considered here, in agreement with available reference values. The predicted binding energies are roughly 25% lower than those from SCAN+rVV10 (revised Vydrov and Van Voorhis 2010), and $\ensuremath{\sim}15%$ lower than those from PBE+rVV10. Since SCAN+rVV10 usually overbinds, the predicted binding energies by SCAN-vdW-dZK and PBE-vdW-dZK are expected to be closer to the true values. The predicted equilibrium binding distances from SCAN-vdW-dZK and PBE-vdW-dZK are slightly larger ($\ensuremath{\sim}0.1\phantom{\rule{0.16em}{0ex}}\AA{}$) than those from SCAN+rVV10, and close to those from SCAN. The binding energy depends upon the number of substrate layers more strongly in vdW-dZK than in rVV10. The ${C}_{4}$-term contributions can be 40% of the total vdW interactions, and the ${C}_{5}$ term contributes about 10%. The effects of images and the back surfaces of slabs can contribute about 4--10%. The vdW interaction energy power laws from the vdW-dZK model for graphene adsorbed on multilayer $\mathrm{Mo}{\mathrm{S}}_{2}$ show slowly varying decay to the pairwise exponent \ensuremath{-}4 with increasing separation $D$, very similar to those obtained from the random-phase approximation and renormalization group approaches by Ambrosetti et al. Both the PBE-vdW-dZK and SCAN-vdW-dZK give a greater increase in interlayer binding energy when the TMD substrate changes from monolayer to four-layer for the graphene/TMD adsorption systems. This is consistent with the relevant electron-energy-loss spectroscopy experimental results showing increased dielectric response from the substrate with increasing substrate layer number.

Directionality in van der Waals Interactions: The Case of 4-Acetylbiphenyl Adsorbed on Au(111)

We report on a theoretical study of adsorption of 4-Acetylbiphenyl molecule and its diffusion properties in the main directions of the Au(111) surface. Structural changes of the molecule, which are induced by adsorption lead to stronger conjugation of the $\pi$-system. The molecule is adsorbed in a flat configuration on the surface with roughly the same binding energy along the [110] and [112] directions, in good agreement with experiments. Furthermore, the diffusion barriers imply an important directionality of the molecule-surface interactions. This is somewhat surprising because our calculations show that the prevailing interaction is the long-range molecule-surface van der Waals interaction. Despite of its weakness, the van der Waals interaction discriminates the preferential adsorption sites as well as imposes a molecular geometry that needs to be considered when rationalizing the diffusion barriers.

Accurate Water Properties from an Efficient ab Initio Method.

Accurate prediction of the water properties from a low-cost ab initio method is still a foremost problem for chemists and physicists. Although density functional approaches starting from semilocal to hybrid exchange-correlation functionals are tested, they are not efficiently performing for all the properties together, especially considering energies, conformal ranking, structures, and dynamics of water. Also, the inclusion of the long-range van der Waals interaction does not improve the ordering stability of isomers. However, relying on the simple revision of the Tao-Mo (revTM) semilocal meta-generalized gradient approximation exchange-correlation functional, we demonstrate that many properties of the water clusters and ice phases can be accurately predicted. A consistent improvement over several popular ab initio methods is achieved, indicating the accuracy of this method for describing hydrogen bonding of water.

Long-range dispersion-corrected density functional for noncovalent interactions

Noncovalent interactions are important in determining structures and properties of molecular complexes and biological molecules, and for understanding adsorption processes in chemistry and biological science, and are still challenging to conventional density functional theories. In this work, the recently developed Tao-Mo (TM) meta-GGA (generalized gradient approximation) functional is combined with the D3 scheme of long-range van der Waals (vdW) interaction correction and the parameters of damping function are optimized with the S66[Formula: see text]×[Formula: see text]8 set. The resulting TM-D3 method is applied to the medium-sized molecular set S22 and large size molecular complexes set L7 to calculate intramolecular interaction energies. The TM-D3 method produces the best accuracy for the S22 set with a MAE of 0.2 kcal/mol, improving upon the PBE-D3 (MAE[Formula: see text]=[Formula: see text]0.5 kcal/mol), PBE0-D3 (MAE[Formula: see text]=[Formula: see text]0.5 kcal/mol), TPSS-D3 (MAE[Formula: see text]=[Formula: see text]0.4 kcal/mol), M06L (MAE[Formula: see text]=[Formula: see text]0.8 kcal/mol), and SCAN-D3 (MAE[Formula: see text]=[Formula: see text]0.4 kcal/mol) methods. For the large size set L7, the TM-D3 (MAE[Formula: see text]=[Formula: see text]2.1 kcal/mol) also performs better than the PBE-D3 (MAE[Formula: see text]=[Formula: see text]2.6 kcal/mol), SCAN-D3 (2.5 kcal/mol) and M06L (3.0 kcal/mol), but not accurate than the PBE0-D3 (MAE[Formula: see text]=[Formula: see text]0.8 kcal/mol) and TPSS-D3 (MAE[Formula: see text]=[Formula: see text]1.1 kcal/mol). However, overall, the TM-D3 method performs very well with an error of 2.7% of mean binding of the S22 set and an error of 12.6% of the mean binding of the L7 set for the two typical and important medium and large size molecular complex sets. The success of the dispersion-corrected TM functional benefits from the ability of the plain TM functional to capture the short-range vdW interaction or extend the short-range interaction to the middle range, and the right coupling between the TM and the long-range vdW correction D3 scheme, leading to the improved description of noncovalent interactions.

Electronic structure and dielectric function of Mn-Bi-Te layered compounds

A comparative study of the electronic and optical properties of Mn-Bi-Te layered compounds was carried out using spectroscopic ellipsometry (SE) over a photon energy range of 0.7–6.5 eV at room temperature and density functional theory (DFT)-based first-principle calculations within the general gradient approximation with Hubbard like correction (GGA+U) and allowance for a spin-orbital coupling. The total energies of the above compounds in ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations are obtained by taking the long-range van der Waals interaction into account. The stability of the AFM state of MnBi2Te4 and MnBi4Te7 over the corresponding FM counterpart is disclosed. The SE-based and calculated dielectric functions are compared. It is shown that interband optical transitions in the accessed photon energy range mainly occur between Mn 3d + Te 5p states of the valence band and Bi 6p + Te 5p with a small admixture of Mn 3d states of the conduction band.

Van der Waals density functional with corrected C6 coefficients

The non-local van der Waals density functional (vdW-DF) has had tremendous success since its inception in 2004 due to its constraint-based formalism that is rigorously derived from a many-body starting point. However, while vdW-DF can describe binding energies and structures for van der Waals complexes and mixed systems with good accuracy, one long-standing criticism---also since its inception---has been that the $C_6$ coefficients that derive from the vdW-DF framework are largely inaccurate and can be wrong by more than a factor of two. It has long been thought that this failure to describe the $C_6$ coefficients is a conceptual flaw of the underlying plasmon framework used to derive vdW-DF. We prove here that this is not the case and that accurate $C_6$ coefficient can be obtained without sacrificing the accuracy at binding separations from a modified framework that is fully consistent with the constraints and design philosophy of the original vdW-DF formulation. Our design exploits a degree of freedom in the plasmon-dispersion model $\omega_{\mathbf{q}}$, modifying the strength of the long-range van der Waals interaction and the cross-over from long to short separations, with additional parameters tuned_ to reference systems. Testing the new formulation for a range of different systems, we not only confirm the greatly improved description of $C_6$ coefficients, but we also find excellent performance for molecular dimers and other systems. The importance of this development is not necessarily that particular aspects such as $C_6$ coefficients or binding energies are improved, but rather that our finding opens the door for further conceptual developments of an entirely unexplored direction within the exact same constrained-based non-local framework that made vdW-DF so successful in the first place.

Van der Waals Correction to the Physisorption of Graphene on Metal Surfaces

Adsorption is a scientifically and technologically important interfacial phenomenon, which however presents challenges to conventional density functional theory (DFT) due to the long-range van der Waals (vdW) interactions. We have developed a model of long-range vdW correction for physisorption of graphene (G) on metals with the Lifshitz–Zaremba–Kohn second-order perturbation theory, by incorporating dipole- and quadrupole-surface interactions and screening effects. The physisorption energies calculated by the model between graphene and eight metal surfaces (Al, Ni, Co, Pd, Pt, Cu, Ag, and Au), and the adsorption energies for the same G/metal structures from self-consistent DFT PBE (Perdew–Burke–Ernzerhof) calculations, are obtained in a range of distances between G and the metal surfaces. The sum of these two parts is the total adsorption energy as a function of the distance, from which the equilibrium distance and the binding energy are determined simultaneously. The results show high accuracy, with the...

BEEF-vdW+U method applied to perovskites: thermodynamic, structural, electronic, and magnetic properties

The recently developed BEEF-vdW exchange-correlation method provides a reasonably reliable description of both long-range van der Waals interactions and short-range covalent bonding between molecules and surfaces. However, this method still suffers from the excessive electron delocalization that is connected with the self-interaction error and, consequently, the calculated chemical and physical properties such as formation energy and band gap deviate markedly from the experimental values, especially when strongly correlated systems are under investigation. In this contribution, BEEF-vdW+U calculations have been performed to study the thermodynamic, structural, electronic, and magnetic properties of La-based perovskites. An effective interaction parameter and an energy adjustment are determined simultaneously by a mixing GGA and GGA+U method, where the enthalpy or Gibbs free energy of formation of oxides containing a transition metal in different oxidation states are fitted to available experimental data. The is found to have its origin in the fact that the GGA+U method gives rise to the offsets in the total energy that include not only the desired physical correction but also an arbitrary contribution. Calculated results indicate that the BEEF-vdW method provides a more accurate description of the bonding in the O2 molecule than the PBE method and has generally smaller values for the 3d-block transition metals, thereby giving rise to band gaps and magnetic moments that are in better agreement with the experimentally measured values.

Two-to-three dimensional transition in neutral gold clusters: The crucial role of van der Waals interactions and temperature

We predict the structures of neutral gas-phase gold clusters ($Au_n$, $n$ = 5$-$13) at finite temperatures based on free-energy calculations obtained by replica-exchange ab initio molecular dynamics. The structures of neutral $Au_5$$-$$Au_{13}$ clusters are assigned at 100 K based on a comparison of experimental far-infrared multiple photon dissociation spectra performed on Kr-tagged gold clusters with theoretical anharmonic IR spectra and free-energy calculations. The critical gold cluster size where the most stable isomer changes from planar to nonplanar is $Au_{11}$ (capped-trigonal prism, $D_{3h}$) at 100 K. However, at 300 K (i.e., room temperature), planar and nonplanar isomers may coexist even for $Au_8$, $Au_9$, and $Au_{10}$ clusters. Density-functional theory exchange-correlation functionals within the generalized gradient or hybrid approximation must be corrected for long-range van der Waals interactions to accurately predict relative gold cluster isomer stabilities. Our work gives insight into the stable structures of gas-phase gold clusters by highlighting the impact of temperature, and therefore the importance of free-energy over total energy studies, and long-range van der Waals interactions on gold cluster stability.