The van der Waals equation: analytical and approximate solutions

The thermodynamic properties, enthalpy of vaporization, entropy, Helmholtz function, Gibbs function, but especially the heat capacity at constant volume of a van der Waals gas (and liquid) at the phase transition are examined in two different limit approximations. The first limit approximation is at the near-critical temperatures, i.e., for T/T _c → 1, where T _c is the critical temperature, the other limit approximation is at the near-zero temperatures, T→ 0. In these limits, the analytical equations for liquid and gas concentrations at saturated conditions were obtained. Although the heat capacities at constant volume of a van der Waals gas and liquid do not depend on the volume, they have different values and their change during the phase transition was calculated. It should be noticed that for real substances the equations obtained at the near-zero temperature are only valid for T > T _{triple point} and T ≪ T _c , which means that found equations can be used only for substances with T _{triple point} ≪ T _c .     

Accurate description of van der Waals complexes by density functional theory including empirical corrections

An empirical method to account for van der Waals interactions in practical calculations with the density functional theory (termed DFT-D) is tested for a wide variety of molecular complexes. As in previous schemes, the dispersive energy is described by damped interatomic potentials of the form C6R(-6). The use of pure, gradient-corrected density functionals (BLYP and PBE), together with the resolution-of-the-identity (RI) approximation for the Coulomb operator, allows very efficient computations for large systems. Opposed to previous work, extended AO basis sets of polarized TZV or QZV quality are employed, which reduces the basis set superposition error to a negligible extend. By using a global scaling factor for the atomic C6 coefficients, the functional dependence of the results could be strongly reduced. The "double counting" of correlation effects for strongly bound complexes is found to be insignificant if steep damping functions are employed. The method is applied to a total of 29 complexes of atoms and small molecules (Ne, CH4, NH3, H2O, CH3F, N2, F2, formic acid, ethene, and ethine) with each other and with benzene, to benzene, naphthalene, pyrene, and coronene dimers, the naphthalene trimer, coronene. H2O and four H-bonded and stacked DNA base pairs (AT and GC). In almost all cases, very good agreement with reliable theoretical or experimental results for binding energies and intermolecular distances is obtained. For stacked aromatic systems and the important base pairs, the DFT-D-BLYP model seems to be even superior to standard MP2 treatments that systematically overbind. The good results obtained suggest the approach as a practical tool to describe the properties of many important van der Waals systems in chemistry. Furthermore, the DFT-D data may either be used to calibrate much simpler (e.g., force-field) potentials or the optimized structures can be used as input for more accurate ab initio calculations of the interaction energies.     

晶体中原子的平均范德华半径

根据晶体中原子的平均体积数据提出包括全部金属元素的原子平均范德华半径值.与现有几个重要的范德华半径体系进行了初步的比较,指出范德华半径值在应用中值得注意的问题,简要提出了有关范德华半径今后研究的方向.     

晶体范德华半径的70年

对过去70年来发表的稀有气体、非金属元素和金属元素的晶体范德华半径重要数值进行了系统分析和总结.从常用的数值中推荐了最可靠值,并指出有关晶体范德华半径值及其应用中的若干问题,以及有待今后进一步研究的方向.     

3D-Graphene/Boron Nitride-stacking Material: a Fundamental van der Waals Heterostructure

The 3D periodic graphene/h-BN(G/BN) heterostuctures were studied. The stacking forms between the graphene and h-BN layers were discussed. The varieties of the geometric and electronic configurations at the interface between graphene and h-BN layers were also reported. The metal-semiconductor transform of the G/BN material can be achieved by adjusting the stacking form of the h-BN layers or changing the proportion of graphene layers in the unit cell. An electrostatic potential well was found at the interface. Due to the potential well and the only dispersion correlation at the interface, the dielectric constant ε_zz in vertical direction was independent on the variety of the thickness or the proportion of the compositions in an unit cell.     

Calculation of the van der Waals volumes of organic molecules

A new and more precise method is proposed for calculating van der Waals atomic and molecular volumes of organic compounds. The method provides for intersections of three or more spheres at one point of space. Such a possibility is essential for calculating the volumes of sterically overcrowded molecules and of molecules with intramolecular hydrogen bonds. A computer program for IBM PC/AT(XT) is developed. Depending on the atomic environment in the molecule, the average values of the volume increments for atoms C, N, O, H, F, Cl, and S are obtained using the data from the Cambridge Structural Database.N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow 117071. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 922–931, April, 1992.     

Determination of van der Waals forces in monolayer films of lipids & biopolymers. Equation of state for two-dimensional films

Amphiphile molecules are characterized by the dual property arising from the interactions between the apolar [alkyl] and the polar part and the surrounding solvent, i.e., water. In assemblies which amphiphiles form in diverse systems, e.g., micelles, soap bubbles, monolayers or bilayers at interfaces, the attractive forces are attributed to the van der Waals forces. It is not easy to estimate the magnitude of van der Waals forces in some of these systems by any direct method.The magnitude of van der Waals forces in spread monolayers of lipids and biopolymers has been reported to be estimated from experimental data. The magnitude of these forces has been estimated by using an equation of state of a very general form, as delineated herein. In the current literature no such attempt has been reported in the analyses of these monolayers spread on aqueous surfaces. These analyses suggest that the predominant surface forces arise from van der Waals interactions, if the magnitude of electrostatic charge repulsions is weak. The equation-of-state as derived indicates that it is useful in providing information about the molecular interaction in monolayers, for both lipids and biopolymers.     

Novel van der Waals Deep-UV Nonlinear Optical Materials

Van der Waals (vdW) deep-UV (DUV) nonlinear optical (NLO) crystal is an important material system recently developed. Herein, we review its concept and original intention, and then summarized the discovery process of related materials, including the role of A-site cations and the resulting two-/one-dimensional vdW DUV NLO systems. Finally, we evaluate the practical DUV NLO performance and prospected the opportunities and challenges.     

Optimized Liquid-Phase Exfoliation of Magnetic van der Waals Heterostructures: Towards the Single Layer and Deterministic Fabrication of Devices

Van der Waals magnetic materials are promising candidates for spintronics and testbeds for exotic magnetic phenomena in low dimensions. The two-dimensional (2D) limit in these materials is typically reached by mechanically breaking the van der Waals interactions between layers. Alternative approaches to producing large amounts of flakes rely on wet methods such as liquid-phase exfoliation (LPE). Here, we report an optimized route for obtaining monolayers of magnetic cylindrite by LPE. We show that the selection of exfoliation times is the determining factor in producing a statistically significant amount of monolayers while keeping relatively big flake areas (~1 µm²). We show that the cylindrite lattice is preserved in the flakes after LPE. To study the electron transport properties, we have fabricated field-effect transistors based on LPE cylindrite. Flakes are deterministically positioned between nanoscale electrodes by dielectrophoresis. We show that dielectrophoresis can selectively move the larger flakes into the devices. Cylindrite nanoscale flakes present a p-doped semiconducting behaviour, in agreement with the mechanically exfoliated counterparts. Alternating current (AC) admittance spectroscopy sheds light on the role played by potential barriers between different flakes in terms of electron transport properties. The present large-scale exfoliation and device fabrication strategy can be extrapolated to other families of magnetic materials.     

Designed Synthesis of van der Waals Heterostructures: The Power of Kinetic Control

Selecting specific 2D building blocks and specific layering sequences of van der Waals heterostructures should allow the formation of new materials with designed properties for specific applications. Unfortunately, the synthetic ability to prepare such structures at will, especially in a manner that can be manufactured, does not exist. Herein, we report the targeted synthesis of new metal–semiconductor heterostructures using the modulated elemental‐reactant technique to nucleate specific 2D building blocks, control their thickness, and avoid epitaxial structures with long‐range order. The building blocks, VSe₂ and GeSe₂, have different crystal structures, which inhibits cation intermixing. The precise control of this approach enabled us to synthesize heterostructures containing GeSe₂ monolayers alternating with VSe₂ structural units with specific sequences. The transport properties systematically change with nanoarchitecture and a charge‐density wave‐like transition is observed.     

Efficient optimization of van der Waals parameters from bulk properties

Due to the computational cost involved, when developing a force field for new compounds, one often avoids fitting van der Waals (vdW) terms, instead relying on a general force field based on the atom type. Here, we provide a novel approach to efficiently optimize vdW terms, based on both ab initio dimer energies and condensed phase properties. The approach avoids the computational challenges of searching the parameter space by using an extrapolation method to obtain a reliable difference quotient for the parameter derivatives based on the central difference. The derivatives are then used in an active‐space optimization method which convergences quadratically. This method is applicable to polarizable and nonpolarizable force fields, although we focus on the parameterization of the AMBER force field. The scaling of the electrostatic potential (ESP) of the compounds is also studied. The algorithm is tested on 12 compounds, reducing the root mean squared error (RMSE) of the density from 0.061 g/cm³ with GAFF parameters to 0.004 g/cm³, and the heat of vaporization from 1.13 to 0.05 kcal/mol. This is done with only four iterations of molecular dynamic runs. Using the optimized vdW parameters, the RMSE of the self‐diffusion was reduced from 1.22 × 10^?9 to 0.78 × 10^?9 m² s^?1 and the RMSE of the hydration free energies was reduced from 0.30 to 0.26 kcal/mol. Scaling the ESP to improve dimer energies resulted in the RMSE improving to 0.77× 10^?9 m² s^?1, but the worsened to 0.33 kcal/mol. © 2013 Wiley Periodicals, Inc.     

Direct Measurement of Repulsive van der Waals Interactions Using an Atomic Force Microscope

The forces of interaction between a flat poly(tetrafluoroethylene) (PTFE) surface and gold spheres (of radii 3–8 μm) were measured as a function of apparent surface separation for different intervening media. For air, fluorinated alkanes, and polar liquids the interaction between the surfaces was found to be attractive. With intervening liquids of low-polarity the interaction was found to be repulsive. This repulsion is attributed to a negative composite Hamaker coefficient leading to van der Waals repulsion.     

Phosphorene-Based van der Waals Heterojunction for Solar Water Splitting

As a clean and renewable future energy source, hydrogen fuel can be produced via solar water splitting. Two-dimensional (2D) black phosphorene (black-P) can harvest visible light due to the desirable band gap, which promises it as a metal-free photocatalyst. However, black-P can be only used to produce hydrogen since the oxidation potential of water locates lower than the position of the valence band maximum. To improve the photocatalytic performance of black-P, here, using black-P and blue phosphorene (blue-P) monolayers, we propose a 2D van der Waals (vdW) heterojunction. Theoretical results, including the band structures, density of states, Bader charge population, charge density di erence, and optical absorption spectra, clearly reveal that the visible light absorption ability is obviously improved, and the band edge alignment of the proposed vdW heterojunction displays a typical type-II feature to effectively separate the photogenerated carriers. At the same time, the built-in interfacialelectric field prevents the electron-hole recombination. These predictions suggest that the examined phosphorene-based vdW heterojunction is an efficient photocatalyst for solar water splitting.     

Empirical bond-order potential for hydrocarbons: adaptive treatment of van der Waals interactions

Bond-order potentials provide a powerful class of models for simulating chemically reactive systems with classical potentials. In these models, the covalent bonding interactions adapt to the environment, allowing bond strength to change in response to local chemical changes. However, the non-bonded interactions should also adapt in response to chemical changes, an effect which is neglected in current bond-order potentials. Here the AIREBO potential is extended to include adaptive Lennard-Jones terms, allowing the van der Waals interactions to vary adaptively with the chemical environment. The resulting potential energy surface and its gradient remain continuous, allowing it to be used for dynamics simulations. This new potential is parameterized for hydrocarbons, and is fit to the energetics and densities of a variety of condensed phase molecular hydrocarbons. The resulting model is more accurate for modeling aromatic and other unsaturated hydrocarbon species, for which the original AIREBO potential had some deficiencies. Testing on compounds not used in the fitting procedure shows that the new model performs substantially better in predicting heats of vaporization and pressures (or densities) of condensed-phase molecular hydrocarbons.     

van der Waals气体状态方程对于实际气体pV_m-p曲线的解释

为加深对物理化学中实际气体行为的认识,通过van der Waals方程对不同温度下实际气体的pV_m-p曲线进行了解释。     

Van der Waals radii of metals from spectroscopic data

The lack of information about the van der Waals radii of metals can be compensated for by using the results of spectroscopic investigations of van der Waals molecules. It has been shown that the interatomic distances in these molecules obey an additive scheme if one allows for the polarization effects. The van der Waals radii of the alkali metals, Ag, Mg, Zn, Cd, Hg, B, Al, In, and Si, have been determined from the interatomic distances in their heteroatomic molecules with atoms of noble gases. Use of the obtained radii for crystal chemistry is discussed.Translated fromIzyestiya Akademil Nauk. Seriya Khimicheskaya, No. 8, pp. 1374–1378, August, 1994.     

Van der Wals团簇ο-xylene N2的共振双光子电离光谱

The one-color resonant two -photon ionization technique is employed to study jetcooled van der Waals(vdW) complex o-xylene?N2 through the S0-S1 transition around the band. The spectra obtained exhibit rich information about the complex intermolecular vdW vibrational modes. We have tentatively assigned all the observed spectral features. The structure of the complex has been obtained by calculation of the minimum energy structure.     

On the van der Waals interactions and the stability of polypeptide chains in helical conformations

In this work, we aim to investigate the contribution of van der Waals (vdW) interactions to the stability of polypeptides in helical conformations studying infinitely long chains of alanine and glycine with density functional theory. To account for vdW interactions, we have used the interatomic pairwise dispersion approach proposed by Tkatchenko–Scheffler (TS), the TS approach with self‐consistent screening (SCS) that self‐consistently includes long‐range electrostatic effects (TS + SCS), the D2 and D3 methods of Grimme et al., and the Langreth–Lundqvist procedure that treats nonlocally the correlation part of the approximation to the exchange‐correlation (xc) functional (called DF). First, we have tested the performance of these strategies studying a set of representative hydrogen bonded dimers. Next, we have studied polyalanine and polyglicine in π‐helix, α‐helix, ‐helix, 2₇, and polyproline‐II conformations and in a fully extended structure. We have found that the DF methodology in combination with a modified version for the Becke approximation to the exchange (optB86b), the D2, D3, TS, and TS + SCS strategies in combination with the Perdew–Burke–Ernzerhof approximation to the xc functional, describe fairly well dimer association energies. Furthermore, the DF method and the D2, D3, TS, and TS + SCS strategies predict very similar helical stabilities even though the approximation used in DF for describing the long‐range dispersion interactions is different that the one used in D2/D3 and TS/TS + SCS. We found that the stability doubles for π and α helices if vdW interactions are taken into account. © 2015 Wiley Periodicals, Inc.