Intermolecular Potential Parameters Research Articles

A general approximation method for accurate evaluation of the intermolecular potential parameters by using density functional theory

In this work, a new approach is developed and presented for determining the intermolecular potential parameters from intermolecular interaction energy data obtained using the Density Functional Theory (DFT) method over a different basis set. The proposed method in this work is especially useful for the evaluation of all intermolecular potential parameters of arbitrary atoms and molecules. The method developed in this study can be used for evaluating some thermodynamic properties of molecules. In general, this approach has great advantages for the determination of all types of potential parameters of atoms and molecules. As an application, the Lennard-Jones (12-6) and Morse potential parameters of the Ga(CH3)3 molecule have been determined first by using the basis set with density functional method (DFT/B3LYP) method over the 6-31G basis set. The speed of sound, heat capacities, and second virial coefficient over the Lennard-Jones (L-J) (12-6) potential of Ga(CH3)3 have been calculated. The results obtained for the speed of sound and heat capacities of Ga(CH3)3 are compared with theoretical and experimental data. The results indicate that our data are in good agreement with data from the literature.

A Simple Equation for the Free Energy of Liquids.

A simple, three-parameter equation is proposed for the free energy of classical fluids composed of molecules which are not hydrogen bonded. Agreement with experiment is much the same as has been obtained from more complicated three-parameter equations. For spherical and nearly spherical molecules, semiquantitative relations between intermolecular pair potential parameters and the parameters of the proposed equation were found. For alkanes, relations between the parameters and carbon number were noted. These relationships contribute to the understanding of fluid state properties in terms of the properties of the molecules of which they are composed.

Mode‐to‐Mode Collision Energy Transfer from Vibrationally Excited C6F6 to NO/N2 Mixed Bath with the Development of New Potential Energy Functions

AbstractChemical dynamics simulations are performed to study collisional intermolecular energy transfer (IET) from highly vibrationally excited C6F6 to NO/N2 mixed baths equilibrated at 300 K. Two baths with a respective total of 200 and 1000 molecules are considered. The simulations are performed with very accurate intramolecular and intermolecular potential energy parameters either taken from literature or developed. A new simulation methodology is implemented to prepare a three‐component bath system. There is a rise in temperature during the IET dynamics in the smaller bath. The rotational ΔT is observed as 85 K in this bath and is much higher than 20 K obtained at 600 ps from the large bath simulation. However, both the simulations show less average energy transfer as compared to the one obtained from pure N2 bath simulation done previously [J. Chem. Phys. 2014, 140, 194103]. The deviation is less for the large bath simulation. The rotational degree of freedom for NO is found less effective towards IET compared to N2, whereas, the center‐of‐mass translational and vibrational modes behave similar to those of N2.

Unimolecular dissociation dynamics of C6H6–C6Cl6 complex and the effect of anharmonicity

Chemical dynamics simulations are performed to study the unimolecular dissociation of C6H6–C6Cl6 (Bz-HCB) complex at a temperature range of 1000–2000 K. The intramolecular and intermolecular potential energy parameters are optimized to reproduce the experimental frequencies, equilibrium energy and the minimum energy geometry of the complex. The microcanonical and canonical rate constants are fit to the harmonic classical Rice–Ramsperger–Kassel–Marcus (RRKM) and transition state theory (TST) equations, respectively. The role of anharmonicity on the dissociation of the complex is examined by considering the modified TST equation with the inclusion of an exponential anharmonic correction. The requirement of such a correction in analyzing the dissociation dynamics of Bz-HCB complex is validated by a Monte Carlo based calculation.

Conformal Sites Theory for Adsorbed Films on Energetically Heterogeneous Surfaces

We present a conformal sites theory for a solid substrate whose surface is both geometrically and energetically heterogeneous, and that interacts with an adsorbed film. The theory is based on a perturbation expansion for the grand potential of a real system with a rough surface about that of a reference system with an ideal reference surface, thus mapping the real system onto a much simpler interfacial system. The expansion is in powers of the intermolecular potential parameters, and leads to mixing rules for the potential parameters of the reference system. Grand canonical Monte Carlo (GCMC) simulations for the adsorption of argon at 87.3 K, carbon dioxide at 273 K and water vapor at 298 K on heterogeneous carbon surfaces are investigated to explore the limits of applicability of the theory. Simulation results indicate that the theory works well with typical asymmetry of the potential parameters in the force field. However, care should be taken when applying the theory to strongly associating fluids, and in the low-pressure region where the active surface sites play an important role. The conformal sites theory can be used to predict the adsorption properties, and to characterize the solid substrate by taking advantage of the corresponding states principle. Other possible applications are also discussed.

Energy of the quasi-free electron in CO and HD: Probing intermolecular potentials within the local Wigner-Seitz model.

We present the quasi-free electron energy V0(ρ) in the weakly polar fluids CO and HD from gas to liquid densities ρ, on noncritical isotherms, and at a temperature near the critical isotherm. These results represent the first systematic investigation of V0(ρ) in polar fluids across a broad density range and illustrate that field enhanced photoemission can be used to obtain data in such systems. We show that the local Wigner-Seitz model for V0(ρ), when coupled with thermodynamic data for the fluids, can yield optimized intermolecular potential parameters, as well as the magnitude of the zero kinetic energy electron scattering length.

Thermophysical properties of binary mixtures containing oxygen and noble gases

This work deals with the determination of reliable transport and equilibrium thermophysical properties of binary mixtures of O2 and the noble gases Ar, Kr, and Xe. The second virial coefficients B(T), the viscosities η(T), and the binary diffusion coefficients D12(T) were calculated for each mixture in the temperature range between 200 K and 1000 K using a a temperature dependent (n-6) Lennard-Jones intermolecular interaction potential model. The properties of the mixtures were obtained by using the Lorentz-Berthelot and Hohm-Zarkova-Damyanova mixing rules. Our results were compared to other theoretical and experimental data and acceptable agreement was found in all cases. Fitting formulae for the calculation of the intermolecular potential parameters and equilibrium and transport properties of the examined mixtures are recommended proposed.

Computer simulation study of the nematic–vapour interface in the Gay–Berne model

ABSTRACTWe present computer simulations of the vapour–nematic interface of the Gay–Berne model. We considered situations which correspond to either prolate or oblate molecules. We determine the anchoring of the nematic phase and correlate it with the intermolecular potential parameters. On the other hand, we evaluate the surface tension associated to this interface. We find a corresponding states law for the surface tension dependence on the temperature, valid for both prolate and oblate molecules.

Atomistic clustering-ordering and high-strain deformation of an Al0.1CrCoFeNi high-entropy alloy.

Computational investigations of structural, chemical, and deformation behavior in high-entropy alloys (HEAs), which possess notable mechanical strength, have been limited due to the absence of applicable force fields. To extend investigations, we propose a set of intermolecular potential parameters for a quinary Al-Cr-Co-Fe-Ni alloy, using the available ternary Embedded Atom Method and Lennard-Jones potential in classical molecular-dynamics simulations. The simulation results are validated by a comparison to first-principles Korringa-Kohn-Rostoker (KKR) - Coherent Potential Approximation (CPA) [KKR-CPA] calculations for the HEA structural properties (lattice constants and bulk moduli), relative stability, pair probabilities, and high-temperature short-range ordering. The simulation (MD)-derived properties are in quantitative agreement with KKR-CPA calculations (first-principles) and experiments. We study AlxCrCoFeNi for Al ranging from 0 ≤ x ≤2 mole fractions, and find that the HEA shows large chemical clustering over a wide temperature range for x < 0.5. At various temperatures high-strain compression promotes atomistic rearrangements in Al0.1CrCoFeNi, resulting in a clustering-to-ordering transition that is absent for tensile loading. Large fluctuations under stress, and at higher temperatures, are attributed to the thermo-plastic instability in Al0.1CrCoFeNi.

Vapor–liquid phase equilibrium diagram for uranium hexafluoride (UF6) using simplified temperature dependent intermolecular potential parameters (TDIP)

The properties of uranium hexafluoride (UF6, CAS: 7783-81-5) are of importance to the nuclear industry as a precursor for the enrichment and product spent fuel reprocessing. This work is focused on obtaining the vapor–liquid equilibrium (VLE) curve for UF6 using temperature dependent intermolecular potential parameters (TDIP) as opposed to temperature independent intermolecular potential parameters (TIIP). TDIP indeed improves the simulated vapor–liquid coexistence curve, critical constants, and vapor pressure of UF6. However, both approaches need improvement since the predictions are still far from reproducing experimental saturated liquid density data for UF6.

Interfacial tensions of industrial fluids from a molecular‐based square gradient theory

This work reports a procedure for predicting the interfacial tension (IFT) of pure fluids. It is based on scaling arguments applied to the influence parameter of the van der Waals theory of inhomogeneous fluids. The molecular model stems from the application of the square gradient theory to the SAFT‐VR Mie equation of state. The theory is validated against computer simulation results for homonuclear pearl‐necklace linear chains made up to six Mie (λ − 6) beads with repulsive exponents spanning from λ = 8 to 44 by combining the theory with a corresponding states correlation to determine the intermolecular potential parameters. We provide a predictive tool to determine IFTs for a wide range of molecules including hydrocarbons, fluorocarbons, polar molecules, among others. The proposed methodology is tested against comparable existing correlations in the literature, proving to be vastly superior, exhibiting an average absolute deviation of 2.2%. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1781–1794, 2016

Understanding the Phase Behavior of Tetrahydrofuran + Carbon Dioxide, + Methane, and + Water Binary Mixtures from the SAFT-VR Approach.

The high-pressure phase diagrams of the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) mixtures are examined using the SAFT-VR approach. Carbon dioxide molecule is modeled as two spherical segments tangentially bonded, water is modeled as a spherical segment with four associating sites to represent the hydrogen bonding, methane is represented as an isolated sphere, and tetrahydrofuran is represented as a chain of m tangentially bonded spherical segments. Dispersive interactions are modeled using the square-well intermolecular potential. In addition, two different molecular model mixtures are developed to take into account the subtle balance between water-tetrahydrofuran hydrogen-bonding interactions. The polar and quadrupolar interactions present in water, tetrahydrofuran, and carbon dioxide are treated in an effective way via square-well potentials of variable range. The optimized intermolecular parameters are taken from the works of Giner et al. (Fluid Phase Equil. 2007, 255, 200), Galindo and Blas (J. Phys. Chem. B 2002, 106, 4503), Patel et al. (Ind. Eng. Chem. Res. 2003, 42, 3809), and Clark et al. (Mol. Phys. 2006, 104, 3561) for tetrahydrofuran, carbon dioxide, methane, and water, respectively. The phase diagrams of the binary mixtures exhibit different types of phase behavior according to the classification of van Konynenburg and Scott, ranging from types I, III, and VI phase behavior for the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) binary mixtures, respectively. This last type is characterized by the presence of a Bancroft point, positive azeotropy, and the so-called closed-loop curves that represent regions of liquid-liquid immiscibility in the phase diagram. The system exhibits lower critical solution temperatures (LCSTs), which denote the lower limit of immiscibility together with upper critical solution temperatures (UCSTs). This behavior is explained in terms of competition between the incompatibility with the alkyl parts of the tetrahydrofuran ring chain and the hydrogen bonding between water and the ether group. A minimum number of unlike interaction parameters are fitted to give the optimal representation of the most representative features of the binary phase diagrams. In the particular case of tetrahydrofuran(1) + water(2), two sets of intermolecular potential model parameters are proposed to describe accurately either the hypercritical point associated with the closed-loop liquid-liquid immiscibility region or the location of the mixture lower- and upper-critical end-points. The theory is not only able to predict the type of phase behavior of each mixture, but also provides a reasonably good description of the global phase behavior whenever experimental data are available.

Monte Carlo simulations of H2O–CaCl2 and H2O–CaCl2–CO2 mixtures

Monte Carlo simulations were performed to obtain the phase behavior of binary H2O–CaCl2 and ternary H2O–CaCl2–CO2 mixtures over a range of conditions. The solubility of CO2 in brines plays a key role in determining the amount that can be trapped via geological carbon storage. Isobaric-isothermal and Gibbs ensemble Monte Carlo simulations with several fixed-point charge force field models were used for the calculations of liquid densities and vapor pressures for the binary, and compositions of both phases for the ternary system. We used the SPC and SPC/E models for water; the Åqvist, Deublein et al., and Smith–Dang parameterizations for CaCl2; and the EPM2, Murthy et al., and TraPPE models for CO2. While none of the model combinations were able to reproduce all the properties of interest, we found that some combinations produce accurate descriptions of individual properties. For the binary system, liquid densities are well represented by the SPC/E and Åqvist model combination, and vapor pressures are best described by the SPC and Åqvist model combination. For CO2 solubility in aqueous CaCl2, the combination of SPC, Smith–Dang, and TraPPE models gives the best predictions, but all the models studied show good predictive capabilities, given that no intermolecular potential parameters were optimized in the present study. These results are broadly consistent with previous calculations for the H2O–NaCl–CO2 system; CaCl2 is found to have a stronger salting-out effect than NaCl at the same molality.

Molecular simulation of fluids with non-identical intermolecular potentials: thermodynamic properties of 10-5 + 12-6 Mie potential binary mixtures.

General methods for combining interactions between particles characterised by non-identical intermolecular potentials are investigated. The combination methods are tested by performing molecular dynamics simulations to determine the pressure, energy, isochoric and isobaric heat capacities, thermal expansion coefficient, isothermal compressibility, Joule-Thomson coefficient, and speed of sound of 10-5 + 12-6 Mie potential binary mixtures. In addition to the two non-identical Mie potentials, mixtures are also studied with non-identical intermolecular parameters. The combination methods are compared with results obtained by simply averaging the Mie exponents. When either the energy or size parameters are non-identical, very significant differences emerge in the thermodynamic properties predicted by the alternative combination methods. The isobaric heat capacity is the thermodynamic property that is most affected by the relative magnitude of the intermolecular potential parameters and the method for combining non-identical potentials. Either the arithmetic or geometric combination of potentials provides a simple and effective way of performing simulations involving mixtures of components characterised by non-identical intermolecular potentials, which is independent of their functional form.

Optimization of intermolecular potential parameters for the CO2/H2O mixture.

Monte Carlo simulations in the Gibbs ensemble were used to obtain optimized intermolecular potential parameters to describe the phase behavior of the mixture CO2/H2O, over a range of temperatures and pressures relevant for carbon capture and sequestration processes. Commonly used fixed-point-charge force fields that include Lennard-Jones 12-6 (LJ) or exponential-6 (Exp-6) terms were used to describe CO2 and H2O intermolecular interactions. For force fields based on the LJ functional form, changes of the unlike interactions produced higher variations in the H2O-rich phase than in the CO2-rich phase. A major finding of the present study is that for these potentials, no combination of unlike interaction parameters is able to adequately represent properties of both phases. Changes to the partial charges of H2O were found to produce significant variations in both phases and are able to fit experimental data in both phases, at the cost of inaccuracies for the pure H2O properties. By contrast, for the Exp-6 case, optimization of a single parameter, the oxygen-oxygen unlike-pair interaction, was found sufficient to give accurate predictions of the solubilities in both phases while preserving accuracy in the pure component properties. These models are thus recommended for future molecular simulation studies of CO2/H2O mixtures.

Measurement and correlation of viscosity of HFC227ea and HFC236fa in the vapor phase

The viscosities of HFC227ea and HFC236fa in vapor phase were measured with an oscillating disk viscometer of the Maxwell type at the temperature range from 300 to 370 K and pressures up to 2.0 MPa. The uncertainty of the reported viscosities was estimated to be within ±2.0% with a coverage factor of k = 2. The experimental viscosities at 0.1 MPa were employed to determine the intermolecular potential parameters with the aid of the extended law of corresponding states developed by Kestin et al. In addition, viscosity models in the vapor phase based on the friction theory at high pressures were correlated for both HFC227ea and HFC236fa.

A molecular dynamics study on the liquid SbCl5 and SbF5 using force fields derived from quantum chemical calculations

The simple six-site intermolecular potential functions have been derived to reproduce the results from density functional theory calculations of the interaction energy for rigid SbF5 and SbCl5. These adjusted intermolecular potential parameters (force field parameters) were used in the molecular dynamics (MD) simulations of the liquids SbF5 and SbCl5 to obtain density, internal energy, enthalpy and radial distribution function (RDF) using Lennard–Jones potential at 1 atm. There is a good agreement between the values of density, heat capacity and RDF obtained from MD and those of experimental.

Determining the parameters of the potential of intermolecular interaction by the Zeno line

The method of determination of intermolecular interaction potential parameters by the Zeno line is proposed. The intermolecular interaction of ethane, propane, ethylene, and propylene is described using a model of site-site interactions and Karr-Konowalow potential. It is shown that the parameters of intermolecular interaction for this potential can be determined from a small number of experimental data in the single-phase area. It is noted that the final parameters allow us to describe the thermodynamic properties over a broad range of gas and liquid states with precision acceptable for practical use.

Semiclassical calculations of half-widths and line shifts for transitions in the 30012←00001 and 30013←00001 bands of CO2. III: Self collisions

This paper is the third in a series devoted to accurate semi-empirical calculations of pressure-broadened half-widths, pressure-induced line shifts, and the temperature dependence of the half-widths of carbon dioxide. In this work complex Robert–Bonamy (CRB) calculations were made for transitions in two of the Fermi-tetrad bands for self-collisions, i.e. the CO2–CO2 system. The intermolecular potential (IP) was adjusted to match measurements of the half-width, its temperature dependence, and the line shift. It is shown that small changes in the parameters describing the IP lead to noticeable changes in the line shape parameters and that it is possible to find a set of IP parameters, which, when used in the CRB formalism, yield half-widths, their temperature dependence, and line shifts in excellent agreement with measurement. This work demonstrates that this agreement can be obtained if the atom–atom potential is expanded to high order and rank (here 20 4 4), the real and imaginary (S1 and Im(S2)) components are retained, and the determination of the trajectories is made by solving Hamilton's equations. It was found that the temperature dependence of the half-width is sensitive to the range of temperatures used in the fit and that the vibrational dependence of the line shape parameters for these two bands is very small. Databases of the half-width, its temperature dependence, and the line shift for the atmospheres of Venus (296–700K fit range for the temperature exponents of the half-widths) and Mars (125–296K fit range for the temperature exponents of the half-widths) are provided. The calculations are compared with the measured data for the bands under study.

Semiclassical calculations of half-widths and line shifts for transitions in the 30012←00001 and 30013←00001 bands of CO2 II: Collisions with O2 and air

The complex Robert–Bonamy (CRB) formalism was used to calculate the half-width, its temperature dependence, and the line shift for CO2 for transitions in the 30012←00001 and 30013←00001 bands with O2 as the perturbing gas. The calculations were done for rotational quantum numbers from J=0 to J=120 with no ad hoc scaling of the line shape equations. The intermolecular potential parameters are adjusted on accurate experimental measurements of the half-widths, its temperature dependence, and the pressure-induced line shifts so that a single intermolecular potential reproduces all three parameters. Using the results of this work and previous results for N2-broadening, air-broadening line shape parameters were also determined. The comparison of the CRB calculations with the experimental data available in the literature for the three line shape coefficients demonstrates the quality of the present calculations for the both bands under study.