Ab Initio Intermolecular Potentials Research Articles

PES and transport properties of the He⋯HBr complex from kinetic theory and molecular dynamics simulations.

The ab initio intermolecular potential of the He⋯HBr van der Waals (vdW) complex was calculated at the CCSD(T)/a5zBF level of theory and expanded in terms of the orthogonal Legendre polynomials. The PES then was implemented to calculate the interaction viscosity (η12) and diffusion (D12) coefficients through classical Mason-Monchick approximation (MMA), quantum mechanical close-coupling (CC), and molecular dynamics (MD) simulations. Energy-dependent Senftleben-Beenakker (SB) cross-sections were calculated using rotationally averaged cross-sections, and Boltzmann averaging was used to reveal the temperature dependence of the SB cross-sections over the temperature range of T = 50-1000 K. The calculated transport properties from MMA are in close agreement with the CC results, especially for temperatures lower than T = 900 K. The ab initio potential data then were used to derive LJ (12,6) and Vashishta MD force fields, and the equilibrium MD simulation methods were implemented to extract η12 and D12 coefficients using Einstein formulas. It was found that the Vashishta 3-body interaction potential model shows better accuracy than the LJ (12,6) model in MD simulations of D12. For η12, however, both MD potential models are successful, and an average absolute deviation of lower than 1% was obtained when compared to the quantum mechanical CC method.

Direct calculations of the CH4+CO2 far infrared collision-induced absorption

We present computations, solely using input data from the literature and thus free of any adjusted parameter, of the far infrared collision-induced absorption (CIA) by interacting CH4 and CO2 molecules. They are based on classical molecular dynamics simulations (CMDS) of the rotational and translational motions of the molecules made using an accurate ab initio CH4-CO2 anisotropic intermolecular potential, and on a long-range expansion of the interaction-induced dipole. Various desymmetrization procedures, which all ensure detailed balance of the spectral density function are a posteriori applied to the CMDS results. The comparison with the available measurements, which have been collected at room temperature, shows that a good agreement can be obtained without introducing any ad hoc short-range dipole components, and it enables to point out the limits of some of the desymmetrization procedures. Tests are also made of the so-called "isotropic approximation", which point out its strong limits, since it leads to large underestimations of the CIA, and question previous computations made using an isotropic potential and long-range expansion of the induced dipole complemented by ad hoc contributions at short distances. Finally, the temperature dependence of the CIA is predicted for applications to planetary atmospheres.

An Accurate Model to Calculate CO2 Solubility in Pure Water and in Seawater at Hydrate–Liquid Water Two-Phase Equilibrium

Understanding of CO2 hydrate–liquid water two-phase equilibrium is very important for CO2 storage in deep sea and in submarine sediments. This study proposed an accurate thermodynamic model to calculate CO2 solubility in pure water and in seawater at hydrate–liquid water equilibrium (HLWE). The van der Waals–Platteeuw model coupling with angle-dependent ab initio intermolecular potentials was used to calculate the chemical potential of hydrate phase. Two methods were used to describe the aqueous phase. One is using the Pitzer model to calculate the activity of water and using the Poynting correction to calculate the fugacity of CO2 dissolved in water. Another is using the Lennard–Jones-referenced Statistical Associating Fluid Theory (SAFT-LJ) equation of state (EOS) to calculate the activity of water and the fugacity of dissolved CO2. There are no parameters evaluated from experimental data of HLWE in this model. Comparison with experimental data indicates that this model can calculate CO2 solubility in pure water and in seawater at HLWE with high accuracy. This model predicts that CO2 solubility at HLWE increases with the increasing temperature, which agrees well with available experimental data. In regards to the pressure and salinity dependences of CO2 solubility at HLWE, there are some discrepancies among experimental data. This model predicts that CO2 solubility at HLWE decreases with the increasing pressure and salinity, which is consistent with most of experimental data sets. Compared to previous models, this model covers a wider range of pressure (up to 1000 bar) and is generally more accurate in CO2 solubility in aqueous solutions and in composition of hydrate phase. A computer program for the calculation of CO2 solubility in pure water and in seawater at hydrate–liquid water equilibrium can be obtained from the corresponding author via email.

Ab Initio Intermolecular Potential Energy Surfaces and Cross Second Virial Coefficients for the Dimer N2-NO.

Ab initio intermolecular potential energy surfaces (PES) of N2-NO have been constructed at the level of theory CCSD(T) with the augmented correlation-consistent basis sets aug-cc-pVmZ (with m = 2, 3, 4). The nitrogen in the closed-shell electronic configuration X1Σ+ and nitric oxide in the open-shell electronic configuration A2Σ+ were employed to calculate ab initio intermolecular interaction energies. The two new ab initio 5-site intermolecular pair potentials at the theoretical level CCSD(T)/aug-cc-pVmZ (with m = 4, 24) were developed appropriately and are suitable for N2-NO dimer by using the nonlinear least-squares fitting method combining MIO and Levenberg–Marquardt algorithms. The correlation quality of these two potentials was found to be very good with R2 values in the range of 0.98372 to 0.99775. The cross second virial coefficients B12(T) of the N2-NO dimer were calculated in the temperature range of 100 to 470 K using the two ab initio 5-site potentials. The discrepancies between the calculated results and the experimental data can be acceptable.

Ab initio calculation of rarefied flows of helium-neon mixture: Classical vs quantum scatterings

In order to faithfully simulate rarefied gas flows of light-weight molecules at cryogenic temperatures down to several kelvins, the Boltzmann equation with the differential cross section calculated from the realistic intermolecular potential should be applied. In the present work, the direct simulation Monte Carlo (DSMC) method with ab initio intermolecular potentials is first implemented into the open-source software dsmcFoam+ for the simulation of general rarefied gas flows. Then, Fourier and Couette flows of the helium-neon mixture are studied for the temperature ranging from 10 K to 2000 K, where the differential cross sections calculated from both classical and quantum mechanics have been used. Our simulation results show that the quantum scattering effects on the heat flux and shear stress are non-negligible when the equilibrium temperature is lower than 500 K. Also, for the Fourier flow, the mole fraction distributions calculated from the quantum scattering are significantly different from those of classical scattering.

Revisiting the cobalt(II) hydration from molecular dynamics and X-ray absorption spectroscopy

Solution chemistry of Co(II) is receiving a renewal attention due to the high interest for knowing the speciation in seawater of its Co radioactive isotope which appeared in the Japan sea as a consequence of the Fukushima-Daichii nuclear power plant accident. Experimental EXAFS and XANES spectra of a dilute Co(II) aqueous solution have been recorded and structural data derived from their analysis. Based on QM calculations, an ab-initio intermolecular potential has been generated for the Co(II)–HO interaction using the hydrated ion model that uses a polarisable and flexible solvent description through the MCDHO2 model. Classical molecular dynamics simulations of Co(II) in water have been performed and X-ray Absorption spectra have been simulated and compared with the experimental ones. Energetic, structural, dynamical and spectroscopical properties of the cobalt cation in solution have been computed and compared with previous experimental and theoretical data. These comparisons have assessed the good performance of the developed intermolecular potential.

Thermodynamic Properties of Vapor-Liquid Equilibria from Monte-Carlo Simulation using ab initio Intermolecular Potentials of Systems H2-H2 and F2-F2

Abstract In this work, we have been carried out GEMC-NVT simulations in the temperature range 18 K–32 K for fluid hydrogen and in range 60 K–140 K for fluid fluorine using four our developed ab initio 5-site intermolecular potentials for dimers H2-H2 and F2-F2, respectively. The thermodynamic properties of vapor-liquid equilibria and the critical points of fluids hydrogen and fluorine were calculated with the obtained densities of coexisting phases and vapor pressures. The simulation results drived from ab initio pair potentials were compared with those from ab initio potential plus three-body Axilrod-Teller potential and experimental data as well as those from Monte Carlo simulation using Lennard-Jones potentials, Deiters equation of state (D1-EOS) and Benedict-Webb-Rubin equation of state (EOS) reported in the literature.

Calculation of cross second virial coefficients using ab initio intermolecular potential energy surfaces for dimer H2-N2

The ab initio intermolecular pair potentials of dimer H2-N2 were calculated from all constructed orientations, using the level of the theory CCSD(T) and basis sets aug-cc-pVmZ (m = 2, 3, 4, 23, 24, 34). The complete basis sets limit aug-cc-pVmZ (m = 23, 24, 34) were extrapolated by ab initio interaction energies at the level of the theory CCSD(T) with correlation-consistent basis sets aug-cc-pVmZ (m = 2, 3, 4). Then the quantum mechanical results were used for developing two new 5-site intermolecular potential functions by fitting ab initio intermolecular energies of dimer H2-N2. The ab initio fitted intermolecular potentials are used in standard thermodynamic relationships to get the cross second virial coefficients B12(T) of the hydrogen-nitrogen mixture. The calculated results are in good agreement with the experimental data.

Symmetry-adapted perturbation theory potential for the adenine dimer.

A new intermolecular interaction potential for the adenine dimer has been developed with the help of a combination of symmetry-adapted perturbation theory and density functional theory (DFT-SAPT). Supermolecular intermolecular interaction energy computations on hydrogen-bonded and stacked adenine dimers at B3LYP-D, MP2, SCS-MP2, SCS-MI-MP2 and CCSD(T) levels showed that DFT-SAPT is in a very good agreement with CCSD(T). The developed ab initio intermolecular potential has been used to predict the cluster structure of adenine dimers, trimers and tetramers. These global cluster optimizations reproduced adenine dimers reported in the literature and moreover new low-energy structures were also located. For trimers and tetramers, new hydrogen-bonded and stacked low-energy structures have also been found. The current findings suggest that the new ab initio potential can further be exploited to reveal the structure and energy of much larger supramolecular adenine clusters.

The intermolecular dimer potential for guanine.

The ab initio intermolecular potential of guanine has been developed with the help of a combination of symmetry-adapted perturbation theory and density functional theory (DFT). The resulting potential has been globally optimized to locate the guanine cluster structures up to tetramers. It has been found that the new potential is able to reproduce the known guanine cluster structures, especially the guanine quartet stabilized by Hoogsteen hydrogen bonds, in addition to new low-energy conformers. The performance of the potential was also compared with the AMBER force field as well as DFT-D and MP2 levels of theory. The model potential is in agreement with the ab initio methods and it shows a better performance compared to AMBER. Therefore, it can be further exploited in molecular dynamics or global optimizations to determine the structure and energy of much larger guanine clusters.

Calculation of second virial coefficients using ab initio intermolecular pair potentials for F2-F2 and H2-F2 dimers

The ab initio intermolecular pair potentials of dimers F2-F2 and H2-F2 were calculated from all constructed orientations, using the level of theory CCSD(T) and basis sets aug-cc-pVmZ (m=2, 3, 23). The complete basis set limit aug-cc-pV23Z was extrapolated by ab initio interaction energies at the level of theory CCSD(T) with two basis sets aug-cc-pVmZ (m=2, 3). Then the quantum mechanical results were used for constructing two new 5-site potential functions by fitting ab initio energies of dimers F2-F2 and H2-F2. The correlation between ab initio and the fitted ab initio energies of 5-site pair potentials for dimers F2-F2 and H2-F2 is appeared by fitness values R2 in range 0.99749–0.99997. The fitted potentials were used in standard thermodynamic relations to obtain the second virial coefficients and the results were compared to experimental data.

Molecular simulation of the thermodynamic, structural, and vapor-liquid equilibrium properties of neon.

The thermodynamic, structural, and vapor-liquid equilibrium properties of neon are comprehensively studied using ab initio, empirical, and semi-classical intermolecular potentials and classical Monte Carlo simulations. Path integral Monte Carlo simulations for isochoric heat capacity and structural properties are also reported for two empirical potentials and one ab initio potential. The isobaric and isochoric heat capacities, thermal expansion coefficient, thermal pressure coefficient, isothermal and adiabatic compressibilities, Joule-Thomson coefficient, and the speed of sound are reported and compared with experimental data for the entire range of liquid densities from the triple point to the critical point. Lustig's thermodynamic approach is formally extended for temperature-dependent intermolecular potentials. Quantum effects are incorporated using the Feynman-Hibbs quantum correction, which results in significant improvement in the accuracy of predicted thermodynamic properties. The new Feynman-Hibbs version of the Hellmann-Bich-Vogel potential predicts the isochoric heat capacity to an accuracy of 1.4% over the entire range of liquid densities. It also predicts other thermodynamic properties more accurately than alternative intermolecular potentials.

Intermolecular vibrations of the CO2–CS2 complex: Experiment and theory agree, but understanding remains challenging

The infrared spectrum of the cross-shaped van der Waals complex CO2–CS2 is observed in the region of the CO2 ν3 fundamental band (≈2350cm−1) using a tuneable diode laser to probe a pulsed supersonic slit jet expansion. Two combination bands are assigned, corresponding to the intermolecular torsion and CO2 bend modes, and their positions and rotational structure agree extremely well (<0.1cm−1) with calculations based on a recent ab initio intermolecular potential. The b-type torsional band is well-behaved, but the a-type CO2 bending band is highly unusual, with large shifts between the subband origins for Ka=0, 2, and 4. The shifts may be rationalized as due to tunnelling effects and Ka-dependent perturbations from other intermolecular modes. But even though they are well predicted by our calculations, there is no simple qualitative explanation. The predicted low-lying planar slipped parallel isomer of CO2–CS2 is not observed.

Ab initio Intermolecular Potential Energy Surface and Calculation of Second Virial Coefficients for the Cl2–Cl2 Dimer

The results presented in this paper are the ab initio intermolecular potentials and the second virial coefficient, B 2 ( T ) of the dimer Cl 2 -Cl 2 . These ab initio potentials were proposed by the quantum chemical calculations at high level of theory CCSD (T) with basis sets of Dunning’s valence correlation-consistent aug-cc-pV m Z ( m = 2, 3); these results were extrapolated to complete basis set limit aug-cc-pV23Z. The ab initio energies of complete basis set limit aug-cc-pV23Z resulted from the exponential extrapolation were used to construct the 5-site pair potential functions. The second virial coefficients for this dimer were predicted from those with four-dimensional integration. The second virial coefficients were also corrected to first-order quantum effects. The results turn out to be in good agreement with experimental data, if available, or with those from empirical correlation. The quality of ab initio 5-site potentials proved the reliability for prediction of molecular thermodynamic properties.

A thermodynamic model to predict the aqueous solubility of hydrocarbon mixtures at two-phase hydrate-liquid water equilibrium

Understanding the fate and transport of hydrocarbons and hydrocarbon mixtures in the deep sea and underlying sediments requires accurate determination of the two-phase hydrate (H)-liquid water (Lw) thermodynamic equilibrium in the absence of a free gas phase. In addition to controlling hydrate formation directly from the aqueous phase, the H-Lw equilibrium provides the aqueous solubility of dissolving hydrate. The aqueous solubility of hydrocarbon mixture at H-Lw equilibrium was calculated based on the van der Waals and Platteeuw statistical thermodynamic model and the Holder model. Langmuir constants were calculated from cell potential parameters obtained from ab initio intermolecular potentials and thus the model contains no fitting parameters. The model accurately predicts pure methane, ethane, and propane experimental H-Lw equilibrium solubility data, including new data provided here for methane. We present hydrocarbon solubilities in water and seawater at H-Lw equilibrium at deep sea conditions for both methane and a thermogenic mixture based on the methane–ethane–propane ratio from the 2010 Macondo oil spill. We also present model predictions of the proportion of occupied large and small cages at H-Lw equilibrium for both systems, as well as hydrocarbon ratios in the hydrate phase for the thermogenic mixture.

Transport coefficients of helium-argon mixture based on ab initio potential.

The viscosity, thermal conductivity, diffusion coefficient, and thermal diffusion factor of helium-argon mixtures are calculated for a wide range of temperature and for various mole fractions up to the 12th order of the Sonine polynomial expansion with an ab initio intermolecular potential. The calculated values for these transport coefficients are compared with other data available in the open literature. The comparison shows that the obtained transport coefficients of helium-argon mixture have the best accuracy for the moment.

Ab initio Intermolecular Potential Energy Surface and Calculation of Second Virial Coefficients for the Cl2-Cl2 Dimer

The results presented in this paper are the ab initio intermolecular potentials and the second virial coefficient, B2 (T) of the dimer Cl2-Cl2. These ab initio potentials were proposed by the quantum chemical calculations at high level of theory CCSD (T) with basis sets of Dunning's valence correlation-consistent aug-cc-pVmZ (m = 2, 3); these results were extrapolated to complete basis set limit aug-cc-pV23Z. The ab initio energies of complete basis set limit aug-cc-pV23Z resulted from the exponential extrapolation were used to construct the 5-site pair potential functions. The second virial coefficients for this dimer were predicted from those with four-dimensional integration. The second virial coefficients were also corrected to first-order quantum effects. The results turn out to be in good agreement with experimental data, if available, or with those from empirical correlation. The quality of ab initio 5-site potentials proved the reliability for prediction of molecular thermodynamic properties.

New inversion and ab initio intermolecular potentials for supercritical fluorine: Calculation of some properties and MD simulation

A new pair-potential energy function of fluorine has been determined via the inversion of reduced viscosity collision integrals. The potential reproduces the second virial coefficient, viscosity, and thermal conductivity of supercritical fluorine in good accordance with experimental data. We have also determined the (F2)2 potential using different ab initio methods at different geometries. Molecular dynamics simulation has been also performed to obtain pressure of supercritical fluorine using the inversion pair-potential supplemented by Feynman–Hibbs quantum correction and the weighted ab initio potential using the CCSD(T)/aug-pvdz method. To consider many-body interactions, the simple three-body potential of Hauschild and Prausnitz [7] used with the inversion potential to improve the properties without requiring an expensive three-body calculation. We have also calculated the three-body interactions for (F2)3 system using the ab initio computations at different orientations. Our results indicated that the three-body potentials improve the two-body supercritical values.

Carbon Dioxide Hydrate Phase Equilibrium and Cage Occupancy Calculations Using Ab Initio Intermolecular Potentials

Gas hydrate deposits are receiving increased attention as potential locations for CO2 sequestration, with CO2 replacing the methane that is recovered as an energy source. In this scenario, it is very important to correctly characterize the cage occupancies of CO2 to correctly assess the sequestration potential as well as the methane recoverability. In order to predict accurate cage occupancies, the guest–host interaction potential must be represented properly. Earlier, these potential parameters were obtained by fitting to experimental equilibrium data and these fitted parameters do not match with those obtained by second virial coefficient or gas viscosity data. Ab initio quantum mechanical calculations provide an independent means to directly obtain accurate intermolecular potentials. A potential energy surface (PES) between H2O and CO2 was computed at the MP2/aug-cc-pVTZ level and corrected for basis set superposition error (BSSE), an error caused due to the lower basis set, by using the half counterpoise method. Intermolecular potentials were obtained by fitting Exponential-6 and Lennard-Jones 6-12 models to the ab initio PES, correcting for many-body interactions. We denoted this model as the “VAS” model. Reference parameters for structure I carbon dioxide hydrate were calculated using the VAS model (site–site ab initio intermolecular potentials) as Δμ(w)(0) = 1206 ± 2 J/mol and ΔH(w)(0) = 1260 ± 12 J/mol. With these reference parameters and the VAS model, pure CO2 hydrate equilibrium pressure was predicted with an average absolute deviation of less than 3.2% from the experimental data. Predictions of the small cage occupancy ranged from 32 to 51%, and the large cage is more than 98% occupied. The intermolecular potentials were also tested by calculating the pure CO2 density and diffusion of CO2 in water using molecular dynamics simulations.

Overtone vibrational spectroscopy in H2-H2O complexes: A combined high level theoretical ab initio, dynamical and experimental study

First results are reported on overtone (v(OH) = 2 ← 0) spectroscopy of weakly bound H(2)-H(2)O complexes in a slit supersonic jet, based on a novel combination of (i) vibrationally mediated predissociation of H(2)-H(2)O, followed by (ii) UV photodissociation of the resulting H(2)O, and (iii) UV laser induced fluorescence on the nascent OH radical. In addition, intermolecular dynamical calculations are performed in full 5D on the recent ab initio intermolecular potential of Valiron et al. [J. Chem. Phys. 129, 134306 (2008)] in order to further elucidate the identity of the infrared transitions detected. Excellent agreement is achieved between experimental and theoretical spectral predictions for the most strongly bound van der Waals complex consisting of ortho (I = 1) H(2) and ortho (I = 1) H(2)O (oH(2)-oH(2)O). Specifically, two distinct bands are seen in the oH(2)-oH(2)O spectrum, corresponding to internal rotor states in the upper vibrational manifold of Σ and Π rotational character. However, none of the three other possible nuclear spin modifications (pH(2)-oH(2)O, pH(2)-pH(2)O, or oH(2)-pH(2)O) are observed above current signal to noise level, which for the pH(2) complexes is argued to arise from displacement by oH(2) in the expansion mixture to preferentially form the more strongly bound species. Direct measurement of oH(2)-oH(2)O vibrational predissociation in the time domain reveals lifetimes of 15(2) ns and <5(2) ns for the Σ and Π states, respectively. Theoretical calculations permit the results to be interpreted in terms of near resonant energy levels and intermolecular alignment of the H(2) and H(2)O wavefunctions, providing insight into predissociation dynamical pathways from these metastable levels.