Long-range Van Der Waals Interactions Research Articles

Atomic structure of metal-halide perovskites from first principles: The chicken-and-egg paradox of the organic-inorganic interaction

We have studied the prototype hybrid organic-inorganic perovskite CH3NH3PbI3 and its three close relatives, CH3NH3SnI3, CH3NH3PbCl3 and CsPbI3, using relativistic density function theory. The long-range van der Waals (vdW) interactions were incorporated into the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional using the Tkatchenko-Scheffler pairwise scheme. Our results reveal that hydrogen bonding, which is well described by the PBE functional, plays a decisive role for the structural parameters of these systems, including the position and orientation of the organic cation as well as the deformation of the inorganic framework. The magnitude of the inorganic-framework deformation depends sensitively on the orientation of the organic cation, and directly influences the stability of the hybrid perovskites. Our results suggest that the organic and the inorganic components complement each other: The low symmetry of the organic cation is the origin of the inorganic-framework deformation, which then aids the overall stabilization of the hybrid perovskite structure. This stabilization is indirectly affected by vdW interactions, which lead to smaller unit-cell volumes than in PBE and therefore modulate the interaction between the organic cation and the inorganic framework. The vdW-induced lattice-constant corrections are system dependent and lead to PBE+vdW lattice constants in good agreement with experiment. Further insight is gained by analysing the vdW contributions. In all iodide-based hybrid perovskites the interaction between the organic cation and the iodide anions provides the largest lattice-constant change, followed by iodine-iodine and the organic cation - heavy-metal cation interaction. These corrections follow an almost linear dependence on the lattice constant within the range considered in our study, and are therefore approximately additive.

Atomic-Scale Variations of the Mechanical Response of 2D Materials Detected by Noncontact Atomic Force Microscopy.

We show that noncontact atomic force microscopy (AFM) is sensitive to the local stiffness in the atomic-scale limit on weakly coupled 2D materials, as graphene on metals. Our large amplitude AFM topography and dissipation images under ultrahigh vacuum and low temperature resolve the atomic and moiré patterns in graphene on Pt(111), despite its extremely low geometric corrugation. The imaging mechanisms are identified with a multiscale model based on density-functional theory calculations, where the energy cost of global and local deformations of graphene competes with short-range chemical and long-range van der Waals interactions. Atomic contrast is related with short-range tip-sample interactions, while the dissipation can be understood in terms of global deformations in the weakly coupled graphene layer. Remarkably, the observed moiré modulation is linked with the subtle variations of the local interplanar graphene-substrate interaction, opening a new route to explore the local mechanical properties of 2D materials at the atomic scale.

An Underwater Surface-Drying Peptide Inspired by a Mussel Adhesive Protein.

Water hampers the formation of strong and durable bonds between adhesive polymers and solid surfaces, in turn hindering the development of adhesives for biomedical and marine applications. Inspired by mussel adhesion, a mussel foot protein homologue (mfp3S-pep) is designed, whose primary sequence is designed to mimic the pI, polyampholyte, and hydrophobic characteristics of the native protein. Noticeably, native protein and synthetic peptide exhibit similar abilities to self-coacervate at given pH and ionic strength. 3,4-dihydroxy-l-phenylalanine (Dopa) proves necessary for irreversible peptide adsorption to both TiO2 (anatase) and hydroxyapatite (HAP) surfaces, as confirmed by quartz crystal microbalance measurements, with the coacervate showing superior adsorption. The adsorption of Dopa-containing peptides is investigated by attenuated total reflection infrared spectroscopy, revealing initially bidentate coordinative bonds on TiO2, followed by H-bonded and eventually long-ranged electrostatic and Van der Waals interactions. On HAP, mfp3s-pep-3Dopa adsorption occurs predominantly via H-bond and outer-sphere complexes of the catechol groups. Importantly, only the Dopa-bearing compounds are able to remove interfacial water from the target surfaces, with the coacervate achieving the highest water displacement arising from its superior wetting properties. These findings provide an impetus for developing coacervated Dopa-functionalized peptides/polymers adhesive formulations for a variety of applications on wet polar surfaces.

Communication: Accurate higher-order van der Waals coefficients between molecules from a model dynamic multipole polarizability.

Due to the absence of the long-range van der Waals (vdW) interaction, conventional density functional theory (DFT) often fails in the description of molecular complexes and solids. In recent years, considerable progress has been made in the development of the vdW correction. However, the vdW correction based on the leading-order coefficient C6 alone can only achieve limited accuracy, while accurate modeling of higher-order coefficients remains a formidable task, due to the strong non-additivity effect. Here, we apply a model dynamic multipole polarizability within a modified single-frequency approximation to calculate C8 and C10 between small molecules. We find that the higher-order vdW coefficients from this model can achieve remarkable accuracy, with mean absolute relative deviations of 5% for C8 and 7% for C10. Inclusion of accurate higher-order contributions in the vdW correction will effectively enhance the predictive power of DFT in condensed matter physics and quantum chemistry.

Influence of helical rise on the self-excited oscillation behavior of zigzag @ zigzag double-wall carbon nanotubes

Oscillation behaviors of oscillators consisting of defect-free multi-walled carbon nanotubes (MWCNTs) have been extensively studied, owing to the operating frequency of the nanotubes being able to reach up to gigahertz. However, there exist defects in most carbon nanotubes, which will affect the friction force between the walls of nanotubes. It is therefore critical to investigate the oscillation characteristics of the MWCNT-based oscillators containing a distorted or defective rotating tube, for the design of MWCNTs-based oscillators. Unlike the case in the armchair carbon nanotubes (Zeng Y H, et al. 2016 Nanotechnology 27 95705), the existence of the helical rise in the zigzag-type nanotubes can induce aberrant or defective shell structures. In this paper, the oscillatory behaviors of zigzag@zigzag double-wall carbon nanotubes containing a rotating inner tube with different helical rises are investigated using the molecular dynamics method. In all the simulation modes, the adaptive intermolecular reactive empirical bond order potential is used in this work for both the covalent bond between carbon atoms and the long-range van der Waals interaction of the force field. The perfect zigzag outer tube is assumed to be fixed while the zigzag inner tube is free after it has been rotated by a torque. At the beginning of the simulation, the whole system is heat bathed at a temperature around 300 K for 60 ps, to gently increase the whole system temperature to around 300 K after the energy minimization. The total number of particles, the system volume, and the absolute temperature are kept unchanged for 60 ps. Then we apply a torque of 30 eV to the inner tube under the constant temperature. After the rotation frequency of the inner tube reaches around 300 GHz, we remove the torque of inner tube and let the whole system be under a constant energy condition. The time steps for all simulations are all chosen to be 1 fs. The total time for the simulation is 3000 ps. It is found that the oscillatory behavior of the inner tube is dependent on the helical rise. The simulation results show that the oscillation frequency of the inner tube increases with the length of helical rise increasing. However, as the helical rise is further increased, the oscillation becomes awful because of the breakage of the inner tube with defects. Moreover, the zigzag@zigzag double-wall carbon nanotubes without any helical rise may be used as an ideal rotating actuator because the inner tube can rotate at an approximately constant rotational frequency. The influence of the system temperature on the oscillatory behavior of inner tube with a helical rise of 0.5 nm is also investigated. The results show that the oscillation amplitude of the inner tube increases with temperature increasing, but the oscillation of the inner tube is extremely unstable if the temperature is higher than a critical value.

Ab initio studies on the structure of and atomic interactions in cellulose IIII crystals

The crystal structure of cellulose IIII was analyzed using first-principles density functional theory (DFT). The geometry was optimized using variable-cell relaxation, as implemented in Quantum ESPRESSO. The Perdew–Burke–Ernzerhof (PBE) functional with a correction term for long-range van der Waals interactions (PBE-D) reproduced the experimental structure well. By using the optimized crystal structure, the interactions existed among the cellulose chains in the crystal were precisely investigated using the NBO analysis. The results showed that the weak bonding nature of CH/O and the hydrogen bonding occur among glucose molecules in the optimized crystal structure. To investigate the strength of interaction, dimeric and trimeric glucose units were extracted from the crystal, and analyzed using MP2 ab initio counterpoise methods with BSSE correction. The results estimated the strength of the interactions. That is, the packed chains along with a-axis interacts with weak bonding nature of CH/O and dispersion interactions by −7.50 kcal/mol, and two hydrogen bonds of O2HO2…O6 and O6HO6…O2 connect the neighboring packed chains with −11.9 kcal/mol. Moreover, FMO4 calculation was also applied to the optimized crystal structure to estimate the strength of the interactions. These methods can well estimate the interactions existed in the crystal structure of cellulose IIII.

A computational study on the effect of local curvature on the adsorption of oxygen on single-walled carbon nanotubes

We investigated the effect of local curvature on the adsorption of oxygen on single-walled carbon nanotubes based on density functional theory calculations with van der Waals corrections. The results showed that as the curvature increases, the interaction of the nanotubes with oxygen increases as well. An oxygen atom was strongly chemisorbed on the bridge site of the nanotubes accompanied by a significant transfer of charge from the surface to the oxygen atom. Larger curvature enhanced both the adsorption energy and charge transfer due to the greater strain on the carbon atoms that led to a better interaction with oxygen. The oxygen molecule was physisorbed on the nanotubes with the interaction arising mostly from the long-range van der Waals interactions. The adsorption energy was also enhanced by greater curvature. The results were compared with the flat graphene sheet to confirm the effects of surface curvature.

Properties of Organic Liquids when Simulated with Long-Range Lennard-Jones Interactions.

In order to increase the accuracy of classical computer simulations, existing methodologies may need to be adapted. Hitherto, most force fields employ a truncated potential function to model van der Waals interactions, sometimes augmented with an analytical correction. Although such corrections are accurate for homogeneous systems with a long cutoff, they should not be used in inherently inhomogeneous systems such as biomolecular and interface systems. For such cases, a variant of the particle mesh Ewald algorithm (Lennard-Jones PME) was already proposed 20 years ago (Essmann et al. J. Chem. Phys. 1995, 103, 8577-8593), but it was implemented only recently (Wennberg et al. J. Chem. Theory Comput. 2013, 9, 3527-3537) in a major simulation code (GROMACS). The availability of this method allows surface tensions of liquids as well as bulk properties to be established, such as density and enthalpy of vaporization, without approximations due to truncation. Here, we report on simulations of ≈150 liquids (taken from a force field benchmark: Caleman et al. J. Chem. Theory Comput. 2012, 8, 61-74) using three different force fields and compare simulations with and without explicit long-range van der Waals interactions. We find that the density and enthalpy of vaporization increase for most liquids using the generalized Amber force field (GAFF, Wang et al. J. Comput. Chem. 2004, 25, 1157-1174) and the Charmm generalized force field (CGenFF, Vanommeslaeghe et al. J. Comput. Chem. 2010, 31, 671-690) but less so for OPLS/AA (Jorgensen and Tirado-Rives, Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 6665-6670), which was parametrized with an analytical correction to the van der Waals potential. The surface tension increases by ≈10(-2) N/m for all force fields. These results suggest that van der Waals attractions in force fields are too strong, in particular for the GAFF and CGenFF. In addition to the simulation results, we introduce a new version of a web server, http://virtualchemistry.org, aimed at facilitating sharing and reuse of input files for molecular simulations.

Long-range interactions between polar bialkali ground-state molecules in arbitrary vibrational levels.

We have calculated the isotropic C6 coefficients characterizing the long-range van der Waals interaction between two identical heteronuclear alkali-metal diatomic molecules in the same arbitrary vibrational level of their ground electronic state X(1)Σ(+). We consider the ten species made up of (7)Li, (23)Na, (39)K, (87)Rb, and (133)Cs. Following our previous work [Lepers et al., Phys. Rev. A 88, 032709 (2013)], we use the sum-over-state formula inherent to the second-order perturbation theory, composed of the contributions from the transitions within the ground state levels, from the transition between ground-state and excited state levels, and from a crossed term. These calculations involve a combination of experimental and quantum-chemical data for potential energy curves and transition dipole moments. We also investigate the case where the two molecules are in different vibrational levels and we show that the Moelwyn-Hughes approximation is valid provided that it is applied for each of the three contributions to the sum-over-state formula. Our results are particularly relevant in the context of inelastic and reactive collisions between ultracold bialkali molecules in deeply bound or in Feshbach levels.

Dynamical screening of van der Waals interactions in nanostructured solids: Sublimation of fullerenes.

Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C60). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.

Estimation of the sorption ability of supramolecular hydrogen sponges

A theoretical estimation of the sorption capacity of hydrogen sponges, i.e., materials composed of nanotubes, in the absence of specific sorption centers at the inner surface of the tubes has been made. The estimate is based on the account for a relatively long-range van der Waals interaction between hydrogen and the sorbent material in the form of the Lennard-Jones potential. It is shown that tubes with a radius of R = 2.1–2.3 A and smaller eject hydrogen molecules, while by R < 2.0–2.2 A this barrier substantially exceeds kT, so that hydrogen practically does not penetrate into the tubes. This result has been confirmed experimentally. The tubes with larger radii, as the calculation shows, retract hydrogen. However, even by a maximum retraction of hydrogen molecules (by tube radii of 2.5–2.8 A), in the absence of specific sorption centers at their inner surface, the sorption capability of the sponges (in recalculation into the sorbent mass) does not exceed half a percent. The presence of a periodic supramolecular relief (corrugation) in the tubes, as a rule, results in a reduction of the sorption capability. Thus, the search for and creation of hydrogen sponges in the class of such supramolecular structures are not topical.

First-Principles Study of Redox End Members in Lithium–Sulfur Batteries

The properties of the solid-phase redox end members, α-S, β-S, Li2S, and Li2S2, are expected to strongly influence the performance of lithium–sulfur batteries. Nevertheless, the fundamental thermodynamic and electronic properties of these phases remain poorly understood. From a computational standpoint, the absence of these data can be explained by the omission of long-ranged van der Waals interactions in conventional density functionals; these interactions are essential for describing the molecular-crystal nature of S-based compounds. Here we apply van der Waals augmented density functional theory (vdW-DF), quasi-particle methods (G0W0), and continuum solvation techniques to predict several structural, thermodynamic, spectroscopic, electronic, and surface characteristics of these phases. The stability of the α allotrope of sulfur at low temperatures is confirmed by calculating the sulfur phase diagram. Similarly, the stability of lithium persulfide, Li2S2, a compound whose presence may limit capacity, was assessed by comparing the energies of several hypothetical A2B2 crystal structures. In all cases Li2S2 is predicted to be unstable with respect to a two-phase mixture of Li2S and α-S, suggesting that Li2S2 is a metastable phase. Regarding surface properties, the stable surfaces and equilibrium crystallite shapes of Li2S and α-S were predicted in the presence and absence of a continuum solvation field intended to mimic the effect of a dimethoxyethane (DME)-based electrolyte. In the case of Li2S, the equilibrium crystallites are comprised entirely of stoichiometric (111) surfaces, while for α-S a complex mixture of several facets is predicted. Finally, G0W0 calculations reveal that all of α-S, β-S, Li2S, and Li2S2 are insulators with band gaps greater than 2.5 eV.

Dispersion Corrected DFT Study of Pentacene Thin Films on Flat and Vicinal Au(111) Surfaces

Here we present a density functional theory study of pentacene ultrathin films on flat and vicinal [(455)] Au(111) surfaces. We have performed crystal and electronic structure calculations by using PBE and optB86b-vdW functionals and investigated the effects of long-range van der Waals interactions for different coverages starting from a single isolated molecule up to 4 monolayers of coverage. For an isolated molecule both functionals yield the hollow site as the most stable one with the bridge-60 site being very close in energy in the case of optB86b-vdW. Binding strength of an isolated pentacene on the step edge was found to be much larger than that on the terrace sites. Different experimentally reported monolayer structures were compared and the (6 × 3) unit cell was found to be energetically more stable than the (2 × 2√7) and (2 × √31) ones. For one monolayer films while dispersion corrected calculations favored flat pentacene molecules on terraces, standard (PBE) calculations either found tilted and flat configurations to be energetically similar (on the flat (111) surface) or favored the tilted configuration (on the (455) surface). PDOS calculations performed with the optB86b-vdW functional showed larger dispersion of molecular orbitals over the Au states for the (455) surface when compared with the flat (111) surface, indicating an enhanced charge carrier transport at the pentacene–gold interface in favor of the vicinal surface. Starting with the second monolayer, both functionals favored tilted configurations for both surfaces. Our results underline the importance of the dispersion corrections for the loosely bound systems like pentacene on gold and the role played by step edges in determining the multilayer film structure and charge transfer at the organic molecule–metal interface.

Two atoms scattering at low and cold energies

A modified static-exchange model is developed to study the collision of an atom with another atom. It includes the effect of long-range dipole–dipole van der Waals interaction between two atoms in addition to the exact effect of short-range force due to Coulomb exchange between two system electrons. Both these interactions dominate at colder energies. The system is treated as four-centre problem in the centre-of-mass frame. The present ab-initio model is useful to study the two-atomic collisions at low energies, as well as cold energies. The new code is applied to study the scattering of positronium (Ps) by hydrogen (H), both in their ground states.

Disjoining Pressure, Healing Distance, and Film Height Dependent Surface Tension of Thin Wetting Films

In this work, we simulate the adsorption of wetting liquid argon films on a model substrate. We calculate the disjoining pressure isotherm and show that it is completely dominated by the long-range van der Waals interactions. Thick films exhibit the expected Hamaker power law decay, but a quantitative description of thin films requires consideration of the detailed structure of the adsorbed layer. The spectrum of film height fluctuations is calculated and shown to provide reliable estimates of the disjoining pressure for all films studied. However, it is observed that the full spectrum can only be reproduced provided that we account for a film height dependent surface tension proportional to the derivative of the disjoining pressure. A simple theory is worked out that describes well the observed film height dependence. Having at hand both the surface tension and the disjoining pressure, we calculate the healing distance of the liquid films, which differs from the classical expectation by a constant of the same order of magnitude as the bulk correlation length. We show these findings have important implications on the behavior of adsorbed liquids and determine corrections to the augmented Young–Laplace equation at the subnanometer length scale.

Structural and electronic properties of the P3HT–PCBM dimer: A theoretical Study

A density functional theory (DFT) study of the supramolecular dimer formed by a 8-unit oligomer of the poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is presented. A dispersion-corrected exchange-correlation potential was used to afford for the long-range van der Waals interactions. Our calculations predict two stable isomers in which the P3HT 8-mer forms a U-shaped structure surrounding the PCBM. From time-dependent DFT calculations it was determined that the maximum absorption intensity of the P3HT–PCBM dimer undergoes a blue-shift of about 80nm with respect to the isolated P3HT oligomer in qualitative agreement with the experimental facts.

Experimental evaluation of the gas-phase collision cross-section of effusive Au beam with noble gases: The effect of size and flux

We find that the effusive atomic beam of Au atoms is deflected away by collision with noble gas atoms crossing in a perpendicular geometry with a beam flux of >1 × 1016/cm2s. The ratio of defected Au atoms is found to increase proportional to the flux of noble gases. In addition, the effective cross-section for the collision between Au and noble gases (Ne, Ar, Xe) is measured to increase in an order of Ne < Ar < Xe. As a result of the increased collision probability, the deflection ratio of Au beam in the noble gases is measured to be enhanced for the Au flux in the range of 1 × 1011–1013Au/cm2s. Our results show that the gas-phase collision can be reliably determined by measuring the deflection ratio. The experimentally determined collision cross-section also explains the variation in the deflection ratio among various noble gases and the importance of a long-range van der Waals interaction between Au and noble gases in the deflection efficiency.

Pressure dependent stability and structure of carbon dioxide—A density functional study including long-range corrections

First-principles density functional theory (DFT) is used to study the solid-state modifications of carbon dioxide up to pressures of 60 GPa. All known molecular CO2 structures are investigated in this pressure range, as well as three non-molecular modifications. To account for long-range van der Waals interactions, the dispersion corrected DFT method developed by Grimme and co-workers (DFT-D3) is applied. We find that the DFT-D3 method substantially improves the results compared to the uncorrected DFT methods for the molecular carbon dioxide crystals. Enthalpies at 0 K and cohesive energies support only one possibility of the available experimental solutions for the structure of phase IV: the R3c modification, proposed by Datchi and co-workers [Phys. Rev. Lett. 103, 185701 (2009)]. Furthermore, comparing bulk moduli with experimental values, we cannot reproduce the quite large--rather typical for covalent crystal structures--experimental values for the molecular phases II and III.

Two-dimensional crystals of Rydberg excitations in a resonantly driven lattice gas

The competition between resonant optical excitation of Rydberg states of atoms and their strong, long-range van der Waals interaction results in spatial ordering of Rydberg excitations in a two-dimensional lattice gas, as observed in a recent experiment of Schau{\ss} et al. [Nature 491, 87 (2012)]. Here we use semiclassical Monte Carlo simulations to obtain stationary states for hundreds of atoms in finite-size lattices. We show the formation of regular spatial structures of Rydberg excitations in a system of increasing size, and find highly sub-Poissonian distribution of the number of Rydberg excitations characterized by a large negative value of the Mandel Q parameter which is nearly independent of the system size.

Electronic Structure and van der Waals Interactions in the Stability and Mobility of Point Defects in Semiconductors

We study the role of electronic structure (band gaps) and long-range van der Waals (vdW) interactions on the stability and mobility of point defects in silicon and heavier semiconductors. Density functional theory calculations with hybrid functionals that contain part of the Hartree-Fock exchange energy are essential to achieve a reasonable description of defect electronic levels, leading to accurate defect formation energies. However, these functionals significantly overestimate the experimental migration barriers. The inclusion of screened vdW interactions further improves the description of defect formation energies, significantly changes the barrier geometries, and brings migration barrier heights into close agreement with experimental values. These results suggest that hybrid functionals with vdW interactions can be successfully used for predictions in a broad range of materials in which the correct description of both the electronic structure and the long-range electron correlation is essential.