General order imaginary time correlation functions and imaginary frequency susceptibilities

Certain periodicity properties of general order imaginary time (‘temperature’) correlation functions can be used to represent the n + 1th order van der Waals pair interactions as a multiple Fourier series whose coefficients are nth-order imaginary frequency susceptibilities of the two molecules. This representation is a generalization to higher orders of the standard expression for the second-order van der Waals pair interaction in terms of imaginary frequency polarizabilities. The fourth-order dispersion energy for spherical molecules in their ground state is obtained as a sum of multiple integrals over imaginary frequencies of certain products of dynamic linear and cubic polarizabilities of the two molecules.     

分子动力学模拟研究方解石表面润湿性反转机理

利用分子动力学模拟技术从分子尺度探究方解石表面润湿性反转机理.首先,研究方解石表面润湿性反转过程;而后,从原油分子-方解石表面与原油分子-原油分子/水分子相互作用两个方面系统揭示方解石表面润湿性反转机理.结果:(1)水分子能够驱离方解石表面弱吸附的非极性分子造成润湿性的改变,但不能驱离强吸附的极性分子使润湿性反转难以实现;(2)原油分子极性越强与方解石表面相互作用越强,极性分子与方解石表面之间主要为静电力,非极性分子与方解石表面之间主要为范德华力;(3)原油分子极性越相近分子之间的相互作用越强,分子极性相差越大分子之间的相互作用越弱.非极性分子之间主要是范德华力,极性分子之间主要是静电力;(4)原油分子在方解石表面和水分子的共同作用下形成乙酸-吡啶-水-甲苯-己烷的稳定吸附序列.本研究为靶向提高采收率技术的设计与应用提供理论基础.     

The boundary element approach to Van der Waals interactions

We develop a boundary element method to calculate Van der Waals interactions for systems composed of domains of spatially constant dielectric response of a general boundary shape. We achieve this by rewriting the interaction energy expression presented in Phys. Rev. B, 62 (2000) 6997 exclusively in terms of surface integrals of surface operators. We validate this approach in the Lifshitz case and give numerical results for the interaction of two spheres as well as the van der Waals self-interaction of a uniaxial ellipsoid. Our method is simple to implement and is particularly suitable for a full, non-perturbative numerical evaluation of non-retarded van der Waals interactions between objects of a completely general shape.     

Taking into account universal intermolecular interactions by the Hückel molecular orbital method

We show that interactions between molecules according to a universal mechanism can be interpreted as changes in the Coulomb integrals in their Hamiltonians. Supplementing the ? technique with the assumption that there is a proportionality between the relative change in the π-electronic charge on the atom and the interaction energy causing these changes allows us to calculate the electronic spectra for solvated dye molecules and their van der Waals complexes. We show that in the molecular orbital calculation, a universal interaction function can act as a semiempirical parameter.     

Van der Waals interactions in DFT made easy by Wannier functions

Ubiquitous van der Waals interactions between atoms and molecules are important for many molecular and solid structures. These systems are often studied from first principles using the density functional theory (DFT). However, the commonly used DFT functionals fail to capture the essence of van der Waals effects. Most attempts to correct for this problem have a basic semiempirical character, although computationally more expensive first principles schemes have been recently developed. We here describe a novel approach, based on the use of the maximally localized Wannier functions, that appears to be promising, being simple, efficient, accurate, and transferable (charge polarization effects are naturally included). The results of test applications to small molecules and bulk graphite are presented.     

Interaction of water‐soluble triphenylphosphines with β‐cyclodextrin: a quantum chemistry study

The interaction of β‐cyclodextrin (β‐CD) with meta‐trisulfonated triphenylphosphine derivatives bearing one or two methyl (or methoxy) groups on the aromatic rings has been investigated by PM3 calculations. The results show that phosphine molecules interact with β‐CD having either an unsubstituted sulfophenyl group or a substituted sulfophenyl group at the para and/or meta‐position. The presence of one methyl or methoxy group in the ortho‐position on each aromatic ring prevents the formation of an inclusion complex between meta‐trisulfonated triphenylphosphine derivatives and β‐CD. The deeply included phosphines in the β‐CD cavity show significant van der Waals interactions with β‐CD. These interactions are at the origin of the high association constants between these molecules and β‐CD. Phosphines exhibiting small association constants interact with β‐CD by forming H‐bonds and weak (or null) van der Waals interactions. Copyright © 2011 John Wiley & Sons, Ltd.     

Anomalous character of the interaction of C<Subscript>60</Subscript> molecules with the sulfur-saturated Ta(100) surface

It is shown that, contrary to all previously studied systems, heating to ～800 K in the C₆₀? TaS² monolayer-Ta(100) adsorption system leads to the complete removal of the deposited fullerene molecules. A model is proposed that explains the observed phenomenon by a very weak nonchemisorption interaction between the C₆₀ molecules and the valence-saturated surface of tantalum disulfide that forms layered crystals with van der Waals interaction between layers.     

Langmuir-Blodgett film and nematic interface

The orientation induced by a Langmuir-Blodgett film on a Nematic Liquid Crystal (NLC) is theoretically analyzed. We show that the effective surface energy is due to different contributions connected with steric and van der Waals interactions between the nematic and the solid substrate. The analysis shows that the Langmuir-Blodgett film orientation depends on the surface density of the molecules. The initial homeotropic orientation may become unstable giving rise to a tilted film. The average orientation of the nematic molecules is also analyzed. We show that, in the event in which the steric interaction Nematic-Langmuir-Blodgett film is very large with respect to the dispersion interaction Nematic-Substrate, the nematic orientation coincides with the one of the film. On the contrary, when the two interactions are comparable, the orientation of the two media may differ. In particular, we analyze how the stable orientation depends on the surface molecular density of the film.     

Proton magnetic resonance of ethane in the liquid and gaseous states

The proton chemical shift of ethane is reported, covering for gaseous ethane the density range 0 to 0·5 g cm^-3 and the temperature range 0 to 110°C, and for liquid ethane from the critical temperature down to -55°C. The results are discussed in terms of the van der Waals interaction term σ _w , using a virial expansion including both binary and ternary collisions. A parallel is drawn between the medium shifts and the equation of state for non-ideal gases. Some comments are made regarding (i) the uniqueness of the parameter B for protons in a C-H bond in non-polar molecules, (ii) the neighbour anisotropy contribution σ _a in molecules such as ethane and ethylene, and (iii) the van der Waals gas-to-liquid medium shift, and its temperature dependence, in larger molecules.     

The role of van der Waals interaction in the tilted binding of amine molecules to the Au(111) surface

We present the results of ab initio electronic structure calculations for the adsorption characteristics of three amine molecules on Au(111), which show that the inclusion of van der Waals interactions between the isolated molecule and the surface leads in general to good agreement with experimental data on the binding energies. Each molecule, however, adsorbs with a small tilt angle (between -5 and 9°). For the specific case of 1,4-diaminobenzene (BDA) our calculations reproduce the larger tilt angle (close to 24°) measured by photoemission experiments, when intermolecular (van der Waals) interactions (for about 8% coverage) are included. These results point not only to the important contribution of van der Waals interactions to molecule-surface binding energy, but also that of intermolecular interactions, often considered secondary to that between the molecule and the surface, in determining the adsorption geometry and pattern formation.     

Repulsive van der Waals forces in soft matter: why bubbles do not stick to walls

Measurements of nonequilibrium hydrodynamic interactions between bubbles and solid surfaces in water provide direct evidence that repulsive van der Waals forces of quantum origin control the behavior of liquid films on solids in air. In addition to being the simplest and most universal 3-phase system, the deformable air-water interface greatly enhances the sensitivity of force measurements compared with rigid systems. The strength of the repulsive interaction, controlled by the choice of solid, is sufficient to prevent coalescence (sticking) on separation due to hydrodynamic interactions.     

Echo experiments in a strongly interacting Rydberg gas

When ground state atoms are excited to a Rydberg state, van der Waals interactions among them can lead to a strong suppression of the excitation. Despite the strong interactions the evolution can still be reversed by a simple phase shift in the excitation laser field. We experimentally prove the coherence of the excitation in the strong blockade regime by applying an "optical rotary echo" technique to a sample of magnetically trapped ultracold atoms, analogous to a method known from nuclear magnetic resonance. We additionally measured the dephasing time due to the interaction between the Rydberg atoms.     

Long-range interactions in polymer melts: the anti-Casimir effect

It is well known that small neutral particles normally tend to aggregate due to the van der Waals forces. We discover a new universal long-range interaction between solid objects in polymer media that is directly opposite the van der Waals attraction. The new force could reverse the sign of the net interaction, possibly leading to the net repulsion. This universal repulsion comes from the subtracted soft fluctuation modes, which are not present in the real polymer system, but rather are in its ideal counterpart. The predicted effect has a deep relation to the classical Casimir interactions, providing an unusual example of fluctuation-induced repulsion instead of normal attraction. That is why it is referred to as the anti-Casimir effect. We also find that the correlation function of monomer units in a concentrated solution of infinite polymer chains follows a power-law rather than an exponential decay at large distances.     

Infra-red spectrum,structure and properties of the N2-Ar van der Waals molecule

The first spectroscopic observation of bound N₂-Ar van der Waals molecules has been achieved with a cryogenic long path cell maintained at 87 K. The infra-red spectrum exhibits prominent fine structure near the N₂ stretching frequency which is assigned to hindered internal rotation of N₂ within the weakly bound complex. An analysis of this fine structure yields a T-shaped equilibrium geometry in which the N₂ bond axis is perpendicular to the N₂-Ar van der Waals bond axis. The observed spectrum is shown to be consistent with an internal rotational barrier of 20 cm^-1 (57 cal/mole). Approximately 20 per cent of the bound species are trapped by this rotational barrier and acquire a locked semi-rigid structure. The remaining 80 per cent have ill-defined geometry and undergo hindered internal rotation. The rotational envelope of an infra-red fundamental is analysed to give an estimate of the N₂-Ar bond length as 3·9 Å.     

单壁碳纳米管中暗激子变亮的多体格林函数理论研究

本文发现了吸附在碳纳米管外壁和管内的有机分子可以调控碳纳米管的光学性质. 本文以手性为(7,0)的单壁碳纳米管与有机分子方酸菁和聚噻吩组成的复合物为例,利用从头算多体格林函数理论研究了物理吸附的有机分子对碳纳米管光激发性质的影响. 碳纳米管的E₁₁和E₂₂吸收峰红移了几十个毫电子伏特. 重要的是,碳纳米管与分子之间的弱相互作用使得(7,0)管原来能量低于E₁₁峰的暗激子获得一定的振子强度,成为亮激子,从而在低能量处产生一个较弱的卫星吸收峰. 吸附分子的类型影响该卫星峰的能量. 这些发现可为调控碳纳米管的发光波长和发光效率提供有价值的信息.     

Selective reflection spectroscopy at the interface between a calcium fluoride window and Cs vapour

A special vapour cell has been built, that allows the measurement of the atom–surface van der Waals interaction exerted by a CaF₂ window at the interface with Cs vapour. Mechanical and thermal fragility of fluoride windows make common designs of vapour cells impractical, so that we have developed an all-sapphire sealed cell with an internal CaF₂ window. Although some impurities have remained when filling up the prototype cell, leading to line broadening and a shift, the selective reflection spectrum of the Cs D₁ line (894 nm) makes apparent the weak van der Waals surface interaction. The uncertainties introduced by the effects of these impurities in the van der Waals measurement are nearly eliminated when comparing the selective reflection signal at the CaF₂ interface of interest and at a sapphire window of the same cell. The ratio of the interactions respectively exerted by a sapphire interface and a CaF₂ interface is found to be 0.55±0.25, in good agreement with the theoretical evaluation of ∼0.67. PACS 34.35.+a; 42.62.Fi; 42.70.Km     

Harmonic dynamical behaviour of thallous halides

Harmonic dynamical behaviour of thallous halides (TlCl and TlBr) have been studied using the new van der Waals three-body force shell model (VTSM), which incorporates the effects of the van der Waals interaction along with long-range Coulomb interactions, three-body interactions and short-range second neighbour interactions in the framework of rigid shell model (RSM). Phonon dispersion curves (PDC), variations of Debye temperature with absolute temperature and phonon density of state (PDS) curves have been reported for thallous halides using VTSM. Comparison of experimental values with those of VTSM and TSM are also reported in the paper and a good agreement between experimental and VTSM values has been found, from which it may be inferred that the incorporation of van der Waals interactions is essential for the complete harmonic dynamical behaviour of thallous halides.     

Effect of long-range forces on surface freezing

We examine the effect of long-range van der Waals interactions on surface freezing (SF) in linear hydrocarbon chain molecules, and the wetting criteria of the bulk liquid by the crystalline surface phase. We find that although the effect of van der Waals interactions is small for SF of normal alkanes, it is important for SF of dry and hydrated alcohols. We also find that the long-range interactions should not be ignored in the interpretation of wetting phenomena in recent experimental results. The results are in good agreement with recent experiments.     

Morphology of nanostructures formed on the van der Waals surface of GaSe layered crystals annealed in sulfur vapor

The morphology of nanostructures, which were grown on the (0001) van der Waals surface of layered GaSe crystals and annealed under thermodynamically equilibrium conditions at high pressures of sulfur vapor, has been investigated using atomic force microscopy. The morphology and phase composition of the nanostructures are determined by the deformation and chemical interactions between the vapor phase and the crystal surface and by the thermodynamic conditions of annealing. Elements of nanostructures are formed as small Ga₂S₃ nanocrystallites, pyramidal quantum dots, and quantum rings on the defective (0001) van der Waals surface that contains nanosized indentations (nanocavities) and liquid gallium.