Chemical Accuracy Obtained in an Ab Initio Molecular Dynamics Simulation of a Fluid by Inclusion of a Three-Body Potential

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155

173

Cooperativity in ionic liquids is investigated by means of static quantum chemical calculations. Larger clusters of the dimethylimidazolium cation paired with a chloride anion are calculated within density functional theory combined with gradient corrected functionals. Tests of the monomer unit show that density functional theory performs reasonably well. Linear chain and ring aggregates have been considered and geometries are found to be comparable with liquid phase structures. Cooperative effects occur when the total energy of the oligomer differs from a simple sum of monomer energies. Cooperative effects have been found in the structural motifs examined. A systematic study of linear chains of increasing length ͑up to nine monomer units͒ has shown that cooperativity plays a more important role than expected and is stronger than in water. The Cl¯H distance of the chloride to the most acidic proton increases with an increasing number of monomer units. The average bond distance approaches 218.9 pm asymptotically. The dipole moment grows almost linearly and the dipole moment per monomer unit reaches the asymptotic value of 16.3 D. The charge on the chloride atoms decreases with an increasing chain length. In order to detect local hydrogen bonding in the clusters a new parametrization of the shared-electron number method is introduced. We find decreasing hydrogen bond energies with an increasing cluster size for both the first hydrogen bond to the most acidic proton and the average hydrogen bond.

Section: Introductionmentioning

confidence: 99%

Cooperativity in ionic liquids

Koßmann

Thar

Kirchner

et al. 2006

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155

173

“…45 Although they have calculated the interaction potentials with and without relativistic effects at the same level, the exact amount of the relativistic corrections cannot be estimated because of the different approaches, in which the basis sets were constructed. 45 To compute the thermophysical properties of the rare gases through atomistic simulations, accurate two-body potential is not sufficient and two factors should be taken into account: the nuclear quantum effects for helium and neon, 18,21,46 and the many-body interactions 22,27,29,32,44,47 especially for the larger rare gases. The quantum effects in neon can be computed through the Wigner-Kirkwood expansion method.…”

Section: Interaction Potentialmentioning

confidence: 99%

“…The second factor, the many-body effects have been the subject of several studies in computer simulations of rare gases. 22,23,27,29,32,44,47 In all of these studies, it has been shown that the Axilrod-Teller three-body interaction 64 is an excellent approximation for the manybody effects. Accurate ͑empirical͒ two-body potentials together with different types of three-body interactions were used by Marcelli and Sadus 44 in Gibbs ensemble MC simulations of argon, krypton, and xenon.…”

Section: A Thermodynamic Propertiesmentioning

confidence: 99%

Theory and atomistic simulation of krypton fluid

Nasrabad

2008

An ab initio interaction potential available in literature is scaled via an empirical procedure and used in an extensive computer simulation study to investigate the thermodynamic properties and self-diffusion coefficient of krypton over a wide range of densities and temperatures. The thermodynamic properties of the fluid phase equilibriums are computed utilizing the Gibbs ensemble Monte Carlo simulation technique. The equation of state and the pair correlation function are obtained using the NVT-Monte Carlo simulation method. The time-correlation function formalism of Green-Kubo is applied in molecular dynamics simulations to calculate the self-diffusion coefficient. Furthermore, the modified Cohen-Turnbull theory is employed to determine the self-diffusion coefficient and the mean free volume needed for this purpose is provided via the generic van der Waals theory. The virial minimization method is used to compute the effective diameter and the results are applied within the generic van der Waals theory as the repulsion-attraction splitting distance of the interaction potential. A remarkable agreement is observed between the computed and empirical results for the orthobaric densities, the vapor pressure, the critical point, and the equation of state. A detailed analysis is presented for the calculated self-diffusion coefficient.

“…23 Small deviations occur at very high densities ͑typically around 1%, probably due to missing many-body effects͒ and larger deviations at extremely low temperatures where translational quantum effects become important. Similar results were found for neon by Kirchner et al 24 with a less accurate ab initio potential but explicitly including three-body interactions. Between the extrema, water and neon, we find CO 2 dioxide with no dipole but a strong quadrupole moment.…”

Section: B Pressurementioning

confidence: 99%

Non-Hamiltonian molecular dynamics implementation of the Gibbs ensemble method. II. Molecular liquid-vapor results for carbon dioxide

Bratschi

Huber

Searles

2007

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Liquid-vapor isotopic fractionation factors of diatomic fluids: A direct comparison between molecular simulation and experiment J. Chem. Phys. 125, 034510 (2006) The Gibbs ensemble molecular dynamics algorithm introduced in the preceding paper ͑paper I͒ ͓C. Bratschi and H. Huber, J. Chem. Phys. v 126, 164104 ͑2007͔͒ is applied to two recently published CO 2 ab initio pair potentials, the Bock-Bich-Vogel and symmetry-adapted perturbation theory site-site potentials. The critical properties of these potentials are calculated for the first time. Critical values and points in the single and two-phase zones are compared with Monte Carlo results to demonstrate the accuracy of the molecular dynamics algorithm, and are compared with experiment to test the accuracy of the potentials. Pressure calculations in the liquid, gas, and supercritical states are carried out and are used to explain potential-related effects and systematic discrepancies. The best ab initio potential yields results in good agreement with experiment.