Equilibrium Distance Research Articles

A study of Ar-N2 supercritical mixtures using neutron scattering, molecular dynamics simulations and quantum mechanical scattering calculations

The microscopic structure of Ar-N2 supercritical mixtures was obtained using neutron scattering experiments at temperatures between 128.4 and 154.1 K, pressures between 48.7 and 97.8 bar and various mole fractions. Molecular Dynamics simulations (MD) were used to study the thermodynamics, microscopic structure and single molecule dynamics at the same conditions. The agreement between experimental and theoretical results on the intermolecular structure was very good. Furthermore, a new explicitly-correlated coupled cluster potential energy surface was obtained for the Ar-N2 van der Waals complex. The ab initio potential energy surface (PES) was found to be in agreement with the MD interaction potential. The global minimum of the ab initio PES De = 98.66 cm−1 was located at the T-shaped geometry and at the intermolecular equilibrium distance of Re = 7.00a0. The dissociation energy of the complex was determined to be D0 = 76.86 cm−1. Quantum mechanical (QM) calculations on the newly obtained PES were used to provide the bound levels of the complex. Finally, integral and differential QM cross sections in Ar + N2 collisions were calculated at collision energy corresponding to the average temperature of the experiments and at room temperature.

All-Electron Quantum Monte Carlo with Jastrow Single Determinant Ansatz: Application to the Sodium Dimer.

In this work, we report potential energy surfaces (PESs) of the sodium dimer calculated by variational (VMC) and lattice-regularized diffusion Monte Carlo (LRDMC). The VMC calculation is accurate for determining the equilibrium distance and the qualitative shape of the experimental PES. Remarkably, after the application of the LRDMC projection to this single determinant ansatz, namely, the Jastrow Antisymmetrized Geminal Power (JAGP), chemical accuracy (∼1 kcal/mol) is reached in the binding energy, and the obtained equilibrium internuclear distance and harmonic vibrational frequency are in very good agreement with the experimental ones. This outcome is crucially dependent on the quality of the optimization used to determine the best possible trial function within the chosen ansatz. The strategy adopted in this work is to minimize the variational energy by initializing the trial function with the density functional theory (DFT) single determinant ansatz expanded exactly in the same atomic basis used for the corresponding VMC and LRDMC calculations. This atomic basis is reshaped ad-hoc for QMC calculations. Indeed, we multiply the standard Gaussian-type atomic orbitals by a one-body Jastrow factor, satisfying, in this way, the electron-ion cusp conditions. In order to achieve these important advantages, we have defined a very efficient DFT algorithm in the mentioned basis, by estimating the corresponding matrix elements on a mesh, and by using a much finer mesh grid in the vicinity of nuclei.

Rydberg states of the CdAr van der Waals complex

A theoretical study of the relatively unexplored area of low-lying Rydberg states of van der Waals molecules (vdW) in the particular case of the CdAr system is presented. The fully ab initio calculation of the potential energy curves (PECs) of the excited vdW complex CdAr above the $(5s6s)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}S_{1},^{\phantom{\rule{0.16em}{0ex}}1}S_{0}$ atomic asymptotes, reaching the $(5s6p)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}P_{0,1,2}, (5s5d)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}D_{2}{,}^{3}{D}_{1,2,3}, (5s6p)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}P_{1}$, and $(5s7s)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}S_{1},^{\phantom{\rule{0.16em}{0ex}}1}S_{0}$, is performed. All the calculated PECs of the Rydberg states of $\mathrm{\ensuremath{\Sigma}}$ symmetry exhibit undulations resulting in the double-well character that was already observed experimentally in the case of the lowest Rydberg states of various 12 group metal--rare gas vdW complexes. The outer wells obtained within the present work are shallow with depths not exceeding 20 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the atomic asymptote and reaching equilibrium distance of roughly 27 bohr in the case of states correlating with the $(5s7s)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}3}S_{1},^{\phantom{\rule{0.16em}{0ex}}1}S_{0}$ asymptotes. The largest calculated values of permanent electric dipoles at equilibrium distances exceed 10 D in the case of ${}^{1}\mathrm{\ensuremath{\Sigma}}$ Rydberg states correlating with $(5s6p)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}P_{1}$ and $(5s5d)\phantom{\rule{0.16em}{0ex}}^{\phantom{\rule{0.16em}{0ex}}1}D_{2}$ atomic asymptotes. The calculations were performed within the second-order multireference perturbation theory. The performance of this particular theoretical ab initio approach is discussed in the context of the considered type of systems and compared, wherever it was possible, with the benchmark results of the coupled-cluster method obtained in this work, with other ab initio calculations based on multireference perturbation theory and experimental data.

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...

Experimental and DFT study on the adsorption of VOCs on activated carbon/metal oxides composites

To enhance the interfacial interaction of activated carbon (AC) with volatile organic compounds (VOCs), the AC surface was modified by depositing metal oxide nanoparticles via an evaporation-induced self-assembly (EISA) method. Taking acetone, toluene and methanol as adsorbates, both dynamic and static VOCs adsorption methods were used to evaluate the adsorption performance of AC and AC/MOx (M = Mg, Zn, Cu and Zr). Compared to pure AC, metal oxide nanoparticles deposition effectively increased the adsorption capacity for acetone and methanol, and AC/ZnO composites showed the best adsorption performance for acetone (415 mg g−1) and methanol (481 mg g−1). In concert with the experiment, a set of systematic theoretical calculations, regarding adsorption energy, adsorption equilibrium distance and charge transfer, were performed to explore the VOCs adsorption mechanism on the metal oxide surface. The adsorption performance can be determined by the Lewis acid-base properties of VOCs-metal oxide systems, the characteristic functional groups and the molecular polarity of VOCs molecules, which paves a new way to develop a novel metal oxide based adsorbent for the VOCs adsorption applications.

Anisotropy of Graphene Nanoflake Diamond Interface Frictional Properties

Using molecular dynamics (MD) simulations, the frictional properties of the interface between graphene nanoflake and single crystalline diamond substrate have been investigated. The equilibrium distance between the graphene nanoflake and the diamond substrate has been evaluated at different temperatures. This study considered the effects of temperature and relative sliding angle between graphene and diamond. The equilibrium distance between graphene and the diamond substrate was between 3.34 Å at 0 K and 3.42 Å at 600 K, and it was close to the interlayer distance of graphite which was 3.35 Å. The friction force between graphene nanoflakes and the diamond substrate exhibited periodic stick-slip motion which is similar to the friction force within a graphene–Au interface. The friction coefficient of the graphene–single crystalline diamond interface was between 0.0042 and 0.0244, depending on the sliding direction and the temperature. Generally, the friction coefficient was lowest when a graphene flake was sliding along its armchair direction and the highest when it was sliding along its zigzag direction. The friction coefficient increased by up to 20% when the temperature rose from 300 K to 600 K, hence a contribution from temperature cannot be neglected. The findings in this study validate the super-lubricity between graphene and diamond and will shed light on understanding the mechanical behavior of graphene nanodevices when using single crystalline diamond as the substrate.

Selenium Vacancy–Enhanced Gas Adsorption of Monolayer Hafnium Diselenide (HfSe2) from a Theoretical Perspective

AbstractThe gas adsorption properties of monolayer selenium vacancy–defective (VSe) hafnium diselenide (HfSe2) compared with that of pure monolayer HfSe2 for CO, NO, NO2, SO2, CO2, H2O, H2S, and NH3 molecules are theoretically investigated using density functional theory (DFT) based on first‐principle calculations. The equilibrium distance, adsorption energy, charge transfer, and electron localization function of pure and Se vacancy–defective HfSe2 (VSe‐HfSe2) monolayers with absorbed gases are systematically calculated. It is demonstrated that monolayer VSe‐HfSe2 exhibits enhanced adsorption energy and charge transfer than that of pure HfSe2, and the band structures reveal that the adsorption of CO, NO, NO2, and SO2 molecules can significantly modify the electronic structure of VSe‐HfSe2 monolayer. In addition, the atom projected density of states suggests the existence of orbital hybridization between the gas molecules, and VSe‐HfSe2 is the primary cause of high charge transfer. The results demonstrate that selenium vacancy will effectively enhance the gas adsorption ability for HfSe2 monolayer, especially for CO, NO, NO2, and SO2 molecules.

Dynamics and spectroscopy of van der Waals complexes composed of ammonia and noble gases.

In this work, we calculate the rovibrational energies and spectroscopic constants for the systems formed by ammonia (NH3) and noble gases (Ng=He, Ne, Ar, Kr and Xe). For the spectroscopic constant calculations, we used two different methods: Dunham and another one that use rovibrational energies (here calculated by discrete variable method). In both cases, we used the improved Lennard-Jones potential energy curves (PECs). These PECs, which describe very well van der Waals systems, were built using the dissociation and equilibrium distance obtained from experiments of crossed molecular beams. The spectroscopic constant results, obtained by both methods were in excellent agreement with each other for all NH3-Ng studied systems. Also in relation to NH3-He system, we realize that although this system has a relatively small dissociation energy, it has one vibrational level. Finally, the spectroscopic constants and fundamental rovibrational energy results were used to verify the stability of each system through the lifetime decomposition.

Performance of Semilocal Kinetic Energy Functionals for Orbital-Free Density Functional Theory

We assess several generalized gradient approximations (GGAs) and Laplacian-level meta-GGAs (LL-MGGA) kinetic energy (KE) functionals for orbital-free density functional theory calculations of bulk metals and semiconductors, considering equilibrium distances, bulk moduli, total and kinetic energies, and the electron densities. We also considered the effects of the pseudopotentials, the vacancy formation energies, and the bond lengths of molecular dimers. We found that LL-MGGA KE functionals are distinctively superior to GGA functionals, showing the importance of the Laplacian of the density in the functional construction. We extended the recently developed Pauli-Gaussian second-order and Laplacian (PGSL) functional ( J. Phys. Chem. Lett. 2018 , 9 , 4385 , DOI: 10.1021/acs.jpclett.8b01926 ) including high-order corrections, achieving higher transferability and accuracy than conventional nonlocal functionals based on the Lindhard response function.

C[sbnd]H versus O[sbnd]H bond scission in methanol decomposition on Pt(111): Role of the dispersion interaction

The dispersion interaction plays an important role in the organic-metal interaction, particularly in the case where the organic molecule is weakly bound to the metal substrate. Here, we report the effects of the dispersion interaction on the equilibrium structure of adsorbed methanol and the energetics of methanol decomposition via the CH and OH bond scissions on Pt(111) based on the results of density functional theory calculations by PBE and optB86b-vdW functionals. The binding mechanism of methanol with the surface is also clarified by the detailed analysis of the density of states, the electron density difference, and the effective Bader charge. The DFT calculated results show that the dispersion interaction significantly reduces the equilibrium distance and increases the adsorption energy of methanol on the top site of Pt(111) to 0.63 eV, thereby making it accord with the experimental result by single-crystal adsorption calorimetry. Moreover, the DFT calculations by PBE functional predict methanol tends to desorb from Pt(111) rather than undergoes decomposition whereas those by optB86b-vdW functional demonstrate that the dispersion interaction facilitates methanol decomposition via the CH bond scission. Our theoretical results provide new insights into the surface chemistry of methanol on Pt(111) under ultrahigh vacuum conditions.

Behavior of H2O molecule in carbon nanotube/boron nitride nanotube heterostructure

We perform total-energy electronic-structure calculations of a water molecule inside a (7, 7) carbon nanotube/boron nitride nanotube (CNT/BNNT) heterojunction. The van der Waals interaction is also considered in this study. We find that the equilibrium distance between the water molecule and the wall of the CNT (BNNT) is ≈ 3.3 Å, and the encapsulation energy is 0.22 eV (0.25 eV). The energy profile along the tube axis exhibits a dramatic change in the vicinity of the heterojunction. A speed change of water flow is expected to occur near the heterojunction. Such information would provide valuable insight in nanostructure design for nanofluidics.

Trapping of Gas Bubbles in Water at a Finite Distance below a Water-Solid Interface.

Gas bubbles in a water-filled cavity move upward because of buoyancy. Near the roof, additional forces come into play, such as Lifshitz, double layer, and hydrodynamic forces. Below uncharged metallic surfaces, repulsive Lifshitz forces combined with buoyancy forces provide a way to trap micrometer-sized bubbles. We demonstrate how bubbles of this size can be stably trapped at experimentally accessible distances, the distances being tunable with the surface material. By contrast, large bubbles (≥100 μm) are usually pushed toward the roof by buoyancy forces and adhere to the surface. Gas bubbles with radii ranging from 1 to 10 μm can be trapped at equilibrium distances from 190 to 35 nm. As a model for rock, sand grains, and biosurfaces, we consider dielectric materials such as silica and polystyrene, whereas aluminium, gold, and silver are the examples of metal surfaces. Finally, we demonstrate that the presence of surface charges further strengthens the trapping by inducing ion adsorption forces.

Theoretical study of the complexes of dichlorobenzene isomers with argon. II. SAPT analysis of the intermolecular interaction

The interaction of argon with dichlorobenzene isomers (DCB-Ar) has been analyzed with the help of the symmetry-adapted perturbation theory based on the density functional description of monomer properties (DFT-SAPT). The global potential energy surface (PES) of these complexes determined from the DFT-SAPT interaction energy (Eint) values has been compared to the CCSD(T) (coupled cluster method including single and double excitations with perturbative triple excitations) PES reported in the companion Paper I [J. Makarewicz and L. Shirkov, J. Chem. Phys. 150, 074301 (2019)]. The equilibrium structures and the binding energies found using DFT-SAPT and CCSD(T) methods combined with adequate basis sets are in good agreement. Besides DCB-Ar, we confirmed that DFT-SAPT gives accurate values of these quantities for other complexes containing an aromatic molecule and Ar. However, DFT-SAPT PES of DCB-Ar is flatter than the corresponding CCSD(T) one. As a result, the intermolecular vibrational energies are systematically underestimated. The analytical form of the important interrelations between SAPT components of Eint, established previously by us [J. Makarewicz and L. Shirkov, J. Chem. Phys. 144, 204115 (2016)], has been approved for the DCB-Ar complexes. Simplified SAPT models based on these relations have been employed to explain physical reasons for differences in the structures and the binding energies of DCB-Ar isomers. It is shown that the equilibrium distance of Ar to DCB plane and the binding energy are determined mainly by dispersion energy. The shift of Ar toward Cl is caused by both exchange and dispersion terms.

Weak Interactions in Alkaline Earth Metal Dimers by Pair-Density Functional Theory

Alkaline earth dimers have small bond energies (less than 5 kcal/mol) that provide a difficult challenge for electronic structure calculations. They are especially challenging for Kohn-Sham density functional theory (KS-DFT) using generalized gradient approximations (GGAs) as the exchange-correlation density functional because GGAs often do not provide accurate results for weak interactions. Here we treat alkaline earth dimers from six different rows of the periodic table. We show that the dominant correlating configurations are the same in all six dimers. We also show that multiconfiguration pair-density functional theory (MC-PDFT) using a fully translated GGA as the on-top density functional not only performs much better than KS-DFT with GGAs in predicting equilibrium distances and dissociation energies but also performs better than the more computationally demanding complete active space second-order perturbation theory (CASPT2) with large basis sets and performs even better than CASPT2 with smaller basis sets.

Molecular dynamics approach for behavior assessment of chitosan nanoparticles in carrying of donepezil and rivastigmine drug molecules

Molecular dynamics (MD) simulation was employed to scrutinize behavior of chitosan nanoparticles (CS-NPs) as a nanocarrier for donepezil and rivastigmine drug molecules. Accordingly, modeling of CS-NPs was carried out based on experimental method (i.e., spontaneous emulsifications). The comparison of the radius of gyration of polymer (Rg) and equilibrium distance (r) of donepezil molecules indicate that CS-NPs is spherically agglomerated before and after addition of ions. However, the behavior of CS-NPs as carrier of rivastigmine molecules changed after addition of ions. The use of CS-NPs length in three dimensions (xyz) and the nearest distance of center of mass (COM) of drug molecules relative to the CS-NPs (rd) demonstrate that entanglement of polymer chains is decreased. Furthermore, behavior of the polymer is confirmed by intra- and/or inter-molecular interactions analysis of components of systems (i.e., CS-NP, drug molecules and ions) and diffusion coefficients of drug molecules. In this regard, the slope of MSD curve versus time is sharply increased in presence of ions in rivastigmine systems due to opening of polymeric chains and release of some of drug molecules. Therefore, drug loading capacity in CS-NPs is decreased in rivastigmine systems relative to donepezil systems. This assessment showed good agreement between results of MD and others experimental work.

An empirical potential energy function for the interactions between rare gas atoms

Abstract A new potential model was developed in this work by the modification of the repulsive part of the Tang-Toennies model. The proposed potential function was used in this work to investigate the interactions between both like and unlike rare gas atoms. The considered systems are He2, Ne2, Ar2, and Kr2 for the like interactions and HeNe, HeAr, and HeKr for the unlike interactions. The potential parameters in the repulsive part of each system were determined from the recent literature values of the equilibrium distance R e , the well depth D e , and the dispersion coefficients C 6 - C 10 . Accurate potential energies available in the literature were compared with the modified potential model and the original Tang-Toennies model. Although both potential energy functions show the same performance in the attractive region and the well-depth region, the proposed potential model can provide a more realistic repulsive wall than the Tang-Toennies model. Since the transport properties of low-density gases are sensitive to the potential energies at small intermolecular distances, the viscosity and thermal conductivity computed in this work were compared with the recommended experimental data in the literature to further verify the quality of the modified potential energy function.

Molecular interaction between asymmetric ligand-capped gold nanocrystals.

Atomistic molecular dynamics simulations are performed to study the potential of mean force (PMF) between two asymmetric gold nanocrystals (NCs) capped by alkylthiols in a vacuum. We systematically investigate the dependence of the PMF on the sizes and capping ligand lengths of two NCs. The results show that the potential well depth scales linearly with increasing total length of two capping ligands on asymmetric dimers, but it hardly depends on the NC size. The predicted equilibrium distance between two asymmetric NCs grows significantly and linearly with the total size of two NCs and exhibits only a slight increase with increasing total ligand length. These findings are explained in terms of the amount of ligand interdigitation between NC surfaces as well as its alterations caused by the change in ligand length and NC size. Furthermore, we introduce a simple formula to estimate the equilibrium distance of two asymmetric NCs. On the basis of the computed PMFs, we propose an empirical two-body potential between asymmetric capped gold NCs.

Hydrogen Bond versus Halogen Bond in HXOn (X = F, Cl, Br, and I) Complexes with Lewis Bases

We have theoretically studied the formation of hydrogen-bonded (HB) and halogen-bonded (XB) complexes of halogen oxoacids (HXOn) with Lewis bases (NH3 and Cl−) at the CCSD(T)/CBS//RIMP2/aug-cc-pVTZ level of theory. Minima structures have been found for all HB and XB systems. Proton transfer is generally observed in complexes with three or four oxygen atoms, namely, HXO4:NH3, HClO3:Cl−, HBrO3:Cl−, and HXO4:Cl−. All XB complexes fall into the category of halogen-shared complexes, except for HClO4:NH3 and HClO4:Cl−, which are traditional ones. The interaction energies generally increase with the number of O atoms. Comparison of the energetics of the complexes indicates that the only XB complexes that are more favored than those of HB are HIO:NH3, HIO:Cl−, HIO2:Cl−, and HIO3:Cl−. The atoms-in-molecules (AIM) theory is used to analyze the complexes and results in good correlations between electron density and its Laplacian values with intermolecular equilibrium distances. The natural bon orbital (NBO) is used to analyze the complexes in terms of charge-transfer energy contributions, which usually increase as the number of O atoms increases. The nature of the interactions has been analyzed using the symmetry-adapted perturbation theory (SAPT) method. The results indicate that the most important energy contribution comes from electrostatics, followed by induction.

Thermodynamic stability of diatomic dications of the families of alkaline earth oxides and hydrides: The cases of BaO2+ and BaH2+

The low-lying electronic states of the diatomic dications BaO2+ and BaH2+ are investigated at a high level of theory. The potential energy curves constructed and the associated spectroscopic parameters provide a global and reliable characterization of these species attesting for their thermodynamic stability. The ground state (X 3Σ−) of BaO2+ is bound (De) by 13.11 kcal mol−1, with an equilibrium distance of 5.526 a0, and the harmonic constant (ωe) equal to 175.1 cm−1. The first excited state (A 3Π), lying higher by 3216 cm−1, is also thermodynamic stable (De = 4.04 kcal mol−1, ωe = 123.7 cm−1). Einstein spontaneous emission coefficients (Av′v″) were evaluated for all band systems, and lifetimes estimated. For the dication BaH2+, only the ground state (X 2Σ+) is thermodynamic stable with De = 5.50 kcal mol−1, Re = 5.444 a0, and ωe = 409.4 cm−1. New experimental results for the mass spectra of BaO2+ are also provided.

Dipole controlled Schottky barrier in the blue-phosphorene-phase of GeSe based van der Waals heterostructures.

Controlling the interface structure is of utmost importance to regulating the nanoscale Schottky barrier height (SBH). Herein, by using first-principles calculations, the electronic properties of the graphene (G) based blue-phosphorene-phase of GeSe van der Waals (vdW) heterostructures, including M/G and X/G interfaces (M = Ge; X = Se), are systematically investigated. When the layer spacing exceeds the vdW gap, n-type Schottky contacts are formed for both MX/G and XM/G heterojunctions. With the layer spacing decreasing to equilibrium distances, due to different charge transfer across the interface, MX/G and XM/G heterojunctions display n- and p-type Schottky contacts, respectively. Further decreasing the layer distance makes both heterojunctions transit into p-type ones. The layer-spacing-dependent SBHs can be rationalized by the increased charge transfer across the interface and the resulting interfacial dipole enhancement. Enlightened by the finding of dipole-controlled SBHs, using MX as building blocks, two different stacking patterns, i.e., nMX-MX-G and nXM-XM-G (n = 1 and 2), are designed to further modulate the SBH. Interestingly, due to the presence of the intrinsic dipole of MX, it is found that the magnitude and orientation of the interfacial dipole can be artificially engineered. With n increasing from 0 to 2, nMX-MX-G with an X/G interface changes from the n-type Schottky contact to Ohmic contact. The Fermi level meets the conduction band and G shows a p-type doping feature finally. Likewise, transition from p-type Schottky contact to Ohmic contact is observed for the nXM-XM-G with M/G interface, accompanied by the Fermi level touching the valence band and the feature of n-type doping for G. The role of nMX stacking seems like the role of applying an external electric field (E-field): applying positive E-field is equivalent to the increase of dipole moment while negative E-field corresponds to the offset of dipole moment. In brief, the SBHs of GeSe/G contact are found to be tunable which originates from the intrinsic dipole of MX. The predictable SBHs for these kinds of charming built-in dipole systems are expected to be highly desirable in electronic devices.