Van Der Waals Potential Energy Research Articles

Excited-state van der Waals potential energy surfaces for the NO A2Σ+ + CO2X1Σg+ collision complex.

Excited state van der Waals (vdW) potential energy surfaces (PESs) of the NO A2Σ+ + CO2X1Σg+ system are thoroughly investigated using coupled cluster theory and complete active space perturbation theory to second order (CASPT2). First, it is shown that pair natural orbital coupled cluster singles and doubles with perturbative triples yields comparable accuracy compared to CCSD(T) for molecular properties and vdW-minima at a fraction of computational cost of the latter. Using this method in conjunction with highly diffuse basis sets and counterpoise correction for basis set superposition error, the PESs for different intermolecular orientations are investigated. These show numerous vdW-wells, interconnected for all geometries except one, with a maximum depth of up to 830 cm-1; considerably deeper than those on the ground state surface. Multi-reference effects are investigated with CASPT2 calculations. The long-range vdW-surfaces support recent experimental observations relating to rotational energy transfer due the anisotropy in the potentials.

Differential Cross Sections for Pair-Correlated Rotational Energy Transfer in NO(A2Σ+) + N2, CO, and O2: Signatures of Quenching Dynamics.

A crossed molecular beam, velocity-map ion-imaging apparatus has been used to determine differential cross sections (DCSs), as a function of collider final internal energy, for rotationally inelastic scattering of NO(A2Σ+, v = 0, j = 0.5f1) with N2, CO, and O2, at average collision energies close to 800 cm-1. DCSs are strongly forward scattered for all three colliders for all observed NO(A) final rotational states, N'. For collisions with N2 and CO, the fraction of NO(A) that is scattered sideways and backward increases with increasing N', as does the internal rotational excitation of the colliders, with N2 having the highest internal excitation. In contrast, the DCSs for collisions with O2 are essentially only forward scattered, with little rotational excitation of the O2. The sideways and backward scattering expected from low-impact-parameter collisions, and the rotational excitation expected from the orientational dependence of published van der Waals potential energy surfaces (PESs), are absent in the observed NO(A) + O2 results. This is consistent with the removal of these short-range scattering trajectories via facile electronic quenching of NO(A) by O2, in agreement with the literature determination of the coupled NO-O2 PESs and the associated conical intersections. In contrast, collisions at high-impact parameter that predominately sample the attractive van der Waals minimum do not experience quenching and are inelastically forward scattered with low rotational excitation.

Total longitudinal potential energy of interaction between parallel charged rods of finite size

We present a formula for the total potential energy of interaction between two parallel charged prismatic rods of finite size as a function of longitudinal inter-rod displacement (TLPE), obtained by calculating the repulsive electrostatic potential energy (using the nonlinear Poisson-Boltzmann equation) and the attractive van der Waals (VDW) potential energy between the rods in skewed configurations. The full profile of the TLPE curves is like an inverted bell, whose (top) width is twice the rod length, and depth is the same as that of the secondary minimum of the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential for the rods.Using the TLPE function, we estimated root mean square of longitudinal displacements of fluctuating rods of 250 × 50 × 50 nm3 and a scaled surface potential of 5 for lateral inter-rod separations, d = 20 and 25 nm, e.g., in 5-rod systems, which showed non-negligible values of 24–87 nm.

Moiré Tuning of the Dynamic Behavior of a Twisted Bilayer van der Waals Material Resonator

Abstract Fully atomistic simulations and a sandwich plate model are used to study the dynamic behavior of twisted 3R-MoS2 bilayers. The simulations demonstrate that for a very small twist angle, the Moiré pattern leads to the symmetry breaking of the interlayer van der Waals energy on the scale of tens of nanometers and causes the dynamic behavior of twisted 3R-MoS2 bilayers to show strong position dependence. In particular, obvious mode pair splitting is observed in twisted 3R-MoS2 bilayer resonators where the interlayer van der Waals energy distribution is nonaxisymmetric. An analysis of the results of these molecular dynamic calculations shows that this behavior can be well explained using the sandwich plate model considering the nonuniform interlayer shear effect. Moreover, the twisted 3R-MoS2 bilayer relaxation mechanism involves the transition from AA stacking order with higher interlayer van der Waals potential energy to AB or BA stacking order, resulting in local buckling in the bilayers. The natural frequencies of resonators dominated by AA domains are much lower than those of resonators dominated by AB domains and even less than those of single-layer 3R-MoS2. Furthermore, as the radius increases, the frequency shows an abnormal trend, and a frequency gap is observed in the resonators dominated by AA domains.

Molecular properties and excited state van der Waals potentials in the NO A2Σ+ + O2 XΣg- collision complex.

We characterize NO A2Σ+ + O2 X3Σg- van der Waals (vdW) Potential Energy Surface (PES) with RHF/RCCSD(T) and CASSCF/CASPT2 calculations. To do this, we first assess our computational setup to properly represent the individual molecular properties of O2 X3Σg-, NO X2Π, and NO A2Σ+. Specifically, we show that highly augmented basis sets are necessary to properly represent the NO A2Σ+ polarizability. Then, we optimize different vdW geometries, and provide BSSE corrected plots of the quartet vdW PES. The surfaces show a confined channel at a distance of approximately 6 Å with a depth of at least 20 cm-1 that we believe is caused by NO A2Σ+ hyper-polarizability. At shorter distances, the channel is connected to a vdW basin centered around the O-N O-O linear geometry with an inter-molecular separation of 4.3 Å, and a depth of 95 cm-1 at the RCCSD(T) level. A CASPT2 scan along the linear geometry show that this vdW basin exists on both the doublet and quartet excited surfaces. These results infer the existence of a collision complex between NO A2Σ+ and O2 X3Σg-, as predicted by earlier experiments.

Simulation of Colloidal Stability and Aggregation Tendency of Magnetic Nanoflowers in Biofluids

A population balance model for the aggregation of iron oxide nanoflowers (IONfs) is presented. The model is based on the fixed pivot technique and is validated successfully for four kinds of aggregation kernels. The extended Derjaguin, Landau, Verwey, and Overbeek (xDLVO) theory is also employed for assessing the collision efficiency of the particles, which is pertinent to the total energy of the interaction. Colloidal stability experiments were conducted on IONfs for two dispersant cases—aqueous phosphate buffered saline solution (PBS) and simulated body fluid (SBF). Dynamic light scattering (DLS) measurements after 24-h of incubation show a significant size increase in plain PBS, whereas the presence of proteins in SBF prevents aggregation by protein corona formation on the IONfs. Subsequent simulations tend to overpredict the aggregation rate, and this can be attributed to the flower-like shape of IONfs, thus allowing patchiness on the surface of the particles that promotes an uneven energy potential and aggregation hindering. In silico parametric study on the effects of the ionic strength shows a prominent dependency of the aggregation rate on the salinity of the dispersant underlying the effect of repulsion forces, which are almost absent in the PBS case, promoting aggregation. In addition, the parametric study on the van der Waals potential energy effect—within common Hamaker-constant values for iron oxides—shows that this is almost absent for high salinity dispersants, whereas low salinity gives a wide range of results, thus underlying the high sensitivity of the model on the potential energy parameters.

Bifurcation study on a degenerate double van der Waals cirque potential energy surface using Lagrangian descriptors

In this paper, we explore the dynamics of a Hamiltonian system after a double van der Waals potential energy surface degenerates into a single well. The energy of the system is increased from the bottom of the potential well up to the dissociation energy, which occurs when the system becomes open. In particular, we study the bifurcations of the basic families of periodic orbits of this system as the energy increases using Lagrangian descriptors and Poincaré maps. We investigate the capability of Lagrangian descriptors to find periodic orbits of bifurcating families for the case of resonant, saddle–node and pitchfork bifurcations.

Method for Evaluating Compatibility between SBS Modifier and Asphalt Matrix Using Molecular Dynamics Models

To further evaluate the compatibility between asphalt matrix and styrene-butadiene-styrene (SBS) modifier using a molecular dynamics (MD) method, MD models of asphalt matrix, SBS modifier, and SBS-modified asphalt were first established. Solubility parameters and interaction energy were utilized to discuss the compatibility between asphalt matrix and SBS modifier. The simulated results indicate that asphalt is compatible with SBS modifier, and the compatibility at 160°C is much better than that at 120°C. The interaction between SBS modifier and asphalt matrix increases their compatibility, and the van der Waals potential energy predominates in the intermolecular potential energy, playing an important role in forming a stable blending system of SBS-modified asphalt. Further, interactions among asphalt components and SBS modifier cannot inhibit the molecular diffusion of each component, lowering the compatibility between asphalt and SBS modifier at 120°C. However, the higher temperature at 160°C inhibits SBS diffusion and increases the interaction and compatibility between SBS and asphalt. This study reveals the segregation mechanism of SBS-modified asphalt.

Transport and roaming on the double van der Waals potential energy surface

This paper explores the phase space structures characterising transport for a double-well van der Waals potential surface. Trajectories are classified as inter-well, intra-well, and escaping-from-a-well to define different dynamical fates. In particular, roaming trajectories, which are a new paradigm in chemical reaction dynamics, are observed. We apply the method of Lagrangian descriptors to uncover the boundaries between the fates. We find that the boundaries between different fates are the stable and unstable manifolds of the hyperbolic periodic orbits, which bifurcate when the energy increases. The methodology presented here applies to other open systems with a double-well potential.

Method to determine van der Waals potential energy of particle interactions for soil clay by dynamic light scattering

AbstractSoil particle interaction forces/energies control aggregation and breakup/dispersion processes, which further profoundly affect a series of soil micro‐ and macro‐processes. For soils with complex compositions, it is the interaction energy of different particles that determines particle aggregation and aggregate breakdown/dispersion. However, measuring the interaction energy for complex systems remains a challenge. In this study we proposed a method to measure this van der Waals potential energy using the dynamic light scattering technique. Firstly, the mathematic relationship between the mean aggregation rate and van der Waals potential energy was established. The mean aggregation rates of particles for either simple or complex systems can be facilely determined by the dynamic light scattering technique. Secondly, for verifying the availability of the suggested method, montmorillonite suspensions in four different electrolyte solutions (NaCl, KCl, MgCl2 and CaCl2) were respectively employed to measure the van der Waals potential energy, which equals −17,817 ± 857 J mol−1 or −7.22 ± 0.304 kT with an average deviation of 4.22%; this directly verified the reliability of the suggested method. Thirdly, the proposed method was also verified indirectly and independently by the ratio of the electrostatic potential energy of particles to the presence of K+ and Na+, as compared with the ratios from the determined van der Waals potential energy of this study and from cation adsorption selectivity. Considering the dynamic light scattering technique has been widely applied for characterizing soil/clay/humus/microbe aggregation, the proposed method should be appropriate for measuring the van der Waals potential energy for all those systems.Highlights Develop a simple method to measure the mean interaction energy for complex systems. The mathematic relationship between the mean aggregation rate and van der Waals energy was established. A method to measure the van der Waals potential energy using the dynamic light scattering technique has been proposed. The van der Waals potential energy in the soil system can be measured in different electrolyte solutions and KCl might be the best one.

Molecular dynamics simulation of weak bonds in carbon fiber reinforced plastic adhesive joints

In carbon fiber-reinforced plastic (CFRP) adhesive joints, poor adhesion because of weak bonds may occur due to contamination at the bond line. We reproduced weak bonds by using a molecular dynamics simulation (MD) approach to investigate the mechanisms of weak bonds. Weak-bond models were created by inserting water (H2O), xylene (C8H10), and silicone molecules as contaminants in the two epoxy models. Tensile analysis was performed to evaluate the mechanical properties of the weak-bond models. The results revealed that the insertion of contaminants reduced the strength of these models. Stress concentration occurred due to the concentration of inserted molecules at the bond line. In addition, the yielding behavior was slower in the H2O model than in the C8H10 and silicone models, and the strength of the H2O model decreased significantly even at a small insertion rate compared with the C8H10 and silicone models. The reason for the difference in yielding behavior may be due to the high diffusion of H2O molecules into the whole epoxy resin. The weak-bond model was reproduced using MD, and the mechanisms of weak bonds were clarified by an analysis of van der Waals potential energy.

The effects of the van der Waals potential energy on the Young’s modulus of a polymer: comparison between molecular dynamics simulation and experiment

Molecular dynamics simulation were employed to investigate the effect of changing the potential energies describing primary and secondary bonds on the Young’s modulus of a polymer. The energies were changed by arbitrarily modifying the parameters of the potential energy model function. The parameters influence the structure of the polymer and its global energy, eventually causing changes to the Young’s modulus. The van der Waals energy describing secondary bonds gives the most significant contribution to the changes. Increasing the energy increases the density and Young’s modulus. The trends are in agreement with experimental data.

Phase space structure and escape time dynamics in a Van der Waals model for exothermic reactions.

We study the phase space objects that control the transport in a classical Hamiltonian model for a chemical reaction. This model has been proposed to study the yield of products in an ultracold exothermic reaction. In this model, two features determine the evolution of the system: a Van der Waals force and a short-range force associated with the many-body interactions. In the previous work, small random periodic changes in the direction of the momentum were used to simulate the short-range many-body interactions. In the present work, random Gaussian bumps have been added to the Van der Waals potential energy to simulate the short-range effects between the particles in the system. We compare both variants of the model and explain their differences and similarities from a phase space perspective. To visualize the structures that direct the dynamics in the phase space, we construct a natural Lagrangian descriptor for Hamiltonian systems based on the Maupertuis action S_{0}=∫_{q_{i}}^{q_{f}}p·dq.

Prediction of methane diffusion coefficient in water using molecular dynamics simulation

Diffusion coefficient is one the most effective factors in mass transfer calculation, which plays an important role in study at the molecular scale. In this study, Material Studio software was used to simulate the diffusion coefficient of methane in water through molecular dynamics. COMPASS force field was also used for optimization of atomic structures of methane and water, and Group-Based method was applied to model to calculate both van der Waals and electrostatic forces. In addition, Universal force field was used to optimize of amorphous cell, while Ewald and Atom-Based methods were applied for modeling and calculation of van der Waals and electrostatic potential energy at constant temperatures. The simulation duration for equilibrium of amorphous cell in both state of NVT and NVE was assumed 5ps. The impact of temperature as well as concentration on diffusion coefficient were investigated and results showed that the diffusion coefficient had linear relationship with temperature and third-degree polynomial relationship with concentration. As a result, of the simulation, the diffusion coefficient function versus temperature and concentration was developed.

Nonadditive properties of van der Waals interactions in a one-dimensional dipole array

The van der Waals potential energy of a linear dipole array is calculated based on a formula using a determinant of a matrix determining the dipole interactions, which includes all orders of the coupling strength and can be applied to a system with an arbitrary number of dipoles. To consider the nonadditivity of the van der Waals interactions, the van der Waals potential energy is expressed as a series of ratios of the susceptibility of the dipole to a cubic root of the distance between the nearest-neighboring dipoles, up to the fourth order for different numbers of dipoles. We show that the contribution of the terms including nonadditive interactions between four dipoles increases with the total number of dipoles and exceeds that between two dipoles. Furthermore, the error in the series expansion is evaluated by comparing the exact potential energy, including all orders of the coupling strength for a small number of dipoles.

Aggregation of polydisperse soil colloidal particles: Dependence of Hamaker constant on particle size

The dispersion and coagulation of soil colloidal particles concern highly with their mobility and activity, as well as the role played in biogeochemical cycle of elements. Particle size is an important factor that affects both the van der Waals potential energy and electrostatic potential energy. However, the size effect of soil particles on surface charge properties and suspension stability has rarely been investigated. Results showed that the zeta potentials (in absolute values) of soil colloidal particles (CP, particle diameter less than 1000 nm) were higher than soil nanoparticles (NP, particle diameter less than 100 nm) for the same solution pH, while the specific surface area of soil NP was 1.6 times of soil CP; taken together, the surface charge density of soil NP was smaller than that of soil CP and the surface charge number of soil NP was slightly higher than soil CP. The stability of soil NP and CP was also different. The critical coagulation concentration (CCC) of soil NP was 1.4 times of soil CP, indicating higher mobility of smaller soil particle in natural conditions. Based on DLVO theory, the Hamaker constants of soil NP and CP were simulated to be 2.06 × 10−20 J and 1.86 × 10−20 J. It could be concluded that the size effect of soil particle influences suspension stability and particle mobility through its effect on Hamaker constant. The results could deepen our understanding for aggregation mechanisms of soil colloid-sized particles and further help in predicting their environmental behaviors.

Study of adhesivity of surfaces using rotational optical tweezers

Optical tweezers are powerful tools for high resolution study of surface properties. Such experiments are traditionally performed by studying the active or the Brownian fluctuation of trapped particles in the X, Y and Z directions. Here we find that employing the fourth dimension, rotation, allows for sensitive and fast probing of the surface, and happen when birefringent microparticles are applied with circularly polarized light, thus called the rotational optical tweezers. When the trapped birefringent microparticle is far away from the surface, the rotation rate is dependent only on the laser power. However, we find that if one traps close to a surface, the rotation rate goes to zero even at finite tweezers laser powers for some specific type of substrates. We suspect this to be due to the interaction between the substrate and the birefringent particle, keeping in mind that the Faxen correction for this mode of rotation cannot increase beyond 1.2 times. We use this to probe some surfaces and find that there is no binding for hydrophobic ones but hydrophilic ones particularly tend to show a laser power threshold to start rotating. We calculate that the threshold energy of the tweezers is consistent with the Van der Waals potential energy, when the mode of interaction with the surface is purely physical. We also find that for chitosan, the mode of interaction is possibly different from Van der Waals. Further, we place the particle on the threshold and observe ‘stick–slip’ kind of rotational behaviour.

Four-way coupling of particle-wall and colloidal particle-particle interactions in direct absorption solar collectors

A novel model based on the Eulerian-Lagrangian approach was introduced in order to capture the physics of particle-fluid, particle-wall, and colloidal particle-particle interactions through four-way coupling in low-flux nanofluid-based direct absorption solar collectors (DASCs). This was accomplished by means of a computationally efficient particle-particle-interaction detection algorithm implemented within the frame of an in-house fluid-particle coupling algorithm that allowed for the simultaneous evolution of the carrier and particulate phases. The optical, thermal, and dispersion effects of particle-wall and colloidal particle-particle interactions were investigated over a range of particle volume fractions ϕp and flow Reynolds numbers (Re). It was found that the outcome of a particle-wall collision is either particle deposition or rebound, depending on the balance between kinetic energy of the incoming particle and particle-wall van der Waals potential energy. Brownian motion was established to be the main deposition mechanism for nanoparticles in the boundary layer. As Re decreased, Brownian motion of nanoparticles dominated over convection, which led to higher deposition rates. Deposited particles were found to cause greater attenuation of solar radiation near the collector upper surface, especially in the visible region of the solar spectrum. Whereas in the infrared region, radiation attenuation became less sensitive to particle deposition. The drop in collector efficiency when particles are allowed to deposit relative to the case when particles are not allowed to deposit was highest for low Re and ϕp. Yet, regardless of Re, an increase in particle surface potential led to a better dispersion of nanoparticles. Furthermore, colloidal interparticle interactions were found to have a negligible effect on collector performance. The results highlight the important effects of particle deposition and non-uniform particle distribution on the performance of DASC systems, which cannot be captured using conventional single-phase models.

Molecular modeling of potential PET imaging agents for adenosine receptor in Parkinson’s disease

The adenosine receptors have appeared as potent and selective drug target in various diseases especially for central nervous system diseases. Adenosine receptor A2A antagonists have been known as potential treatment for Parkinson’s disease (PD). Radiolabeled A2AR antagonists can be used as positron emission tomography (PET) tracers and diagnostic tools for PD. In the present investigation, we perform the quantitative structure–activity relationship (QSAR) analysis and docking studies of a series of PET tracers as ligands and Adenosine receptors (A2AR) binding affinity, to elucidate the structural properties required for A2AR antagonist in treatment for PD. Several variable-selection methods were used to choose the descriptors that would lead to good QSAR model. Among several models developed, the best model was a five-variable multiple linear regression (MLR) equation with statistical parameters of squared correlation coefficient R 2 = 0.90 ± 0.01 and cross-validated correlation coefficient Q 2 = 0.84 ± 0.02. The QSAR models were also constructed for A2AR selectivity to tracer ligands, that yielded a four-variable model with R 2 = 0.94 ± 0.01 and Q 2 = 0.89 ± 0.02. The most important variables contributed in models construction involved: partial charge, hydrophobic atoms, rotatable bonds, polar van der Waals surface area, potential energy, and conformation-dependent charge descriptors. Finally, molecular docking analysis was carried out to better understand the interactions between ligands and Adenosine receptors. The importance of π-π stacking interactions between aromatic moiety of the ligands and triazine core of A2AR antagonist was confirmed.

Van der Waals potential and vibrational energy levels of the ground state radon dimer

In the present paper, the ground state van der Waals potential of the Radon dimer is described by the Tang-Toennies potential model, which requires five essential parameters. Among them, the two dispersion coefficients C6 and C8 are estimated from the well determined dispersion coefficients C6 and C8 of Xe2. C10 is estimated by using the approximation equation that C6C10/C82 has an average value of 1.221 for all the rare gas dimers. With these estimated dispersion coefficients and the well determined well depth De and Re the Born–Mayer parameters A and b are derived. Then the vibrational energy levels of the ground state radon dimer are calculated. 40 vibrational energy levels are observed in the ground state of Rn2 dimer. The last vibrational energy level is bound by only 0.0012cm−1.