1,720,984 research outputs found
Evaluation of B3LYP, X3LYP, and M06-Class Density Functionals for Predicting the Binding Energies of Neutral, Protonated, and Deprotonated Water Clusters
No full textIn this paper we assess the accuracy of the B3LYP, X3LYP, and newly developed M06-L, M06-2X, and M06 functionals to predict the binding energies of neutral and charged water clusters including (H(2)O)(n), n = 2-8, 20), H(3)O(+)(H(2)O)(n), n = 1-6, and OH(-)(H(2)O)(n), n = 1-6. We also compare the predicted energies of two ion hydration and neutralization reactions on the basis of the calculated binding energies. In all cases, we use as benchmarks calculated binding energies of water clusters extrapolated to the complete basis set limit of the second-order Moller-Plesset perturbation theory with the effects of higher order correlation estimated at the coupled-cluster theory with single, double, and perturbative triple excitations in the aug-cc-pVDZ basis set. We rank the accuracy of the functionals on the basis of the mean unsigned error (MUE) between calculated benchmark and density functional theory energies. The corresponding MUE (kcal/mol) for each functional is listed in parentheses. We find that M06-L (0.73) and M06 (0.84) give the most accurate binding energies using very extended basis sets such as aug-cc-pV5Z. For more affordable basis sets, the best methods for predicting the binding energies of water clusters are M06-L/aug-cc-pVTZ (1.24), B3LYP/6-311++G(2d,2p) (1.29), and M06/aug-cc-PVTZ (1.33). M06-L/aug-cc-pVTZ also gives more accurate energies for the neutralization reactions (1.38), whereas B3LYP/6-311++G(2d,2p) gives more accurate energies for the ion hydration reactions (1.69)
Optimization and application of lithium parameters for the reactive force field, ReaxFF
No full textTo make a practical molecular dynamics (MD) simulation of the large-scale reactive chemical systems of Li-H and Li-C, we have optimized parameters of the reactive force field (ReaxFF) for these systems. The parameters for this force field were obtained from fitting to the results of density functional theory (DFT) calculations on the structures and energy barriers for a number of Li-H and Li-C molecules, including Li-2, LiH, Li2H2, H3C-Li, H3C-H2C-Li, H2C=C-LiH, HC&3bond; CLi, H6C5-Li, and Li2C2, and to the equations of state and lattice parameters for condensed phases of Li. The accuracy of the developed ReaxFF was also tested by comparison to the dissociation energies of lithium-benzene sandwich compounds and the collision behavior of lithium atoms with a C-60 buckyball.This research was supported by a grant (code no. 04K1501-02210) from the Center for Nanostructured Materials Technology under 21st Century Frontier R&D Programs of the Ministry of Science and Technology, Korea
Liquefaction of H2 molecules upon exterior surfaces of carbon nanotube bundles
Full text linkWe have used molecular dynamics simulations to investigate interaction of H-2 (molecules on the exterior surfaces of carbon nanotubes (CNTs): single and bundle types. At 80 K and 10 MPa, it is found that charge transfer occurs from a low curvature region to a high curvature region of the deformed CNT bundle, which develops charge polarization only on the deformed structure. The long-range electrostatic interactions of polarized charges on the deformed CNT bundle with hydrogen molecules are observed to induce a high local-ordering of H)(2) (gas that results in hydrogen liquefaction. Our predicted heat of hydrogen liquefaction on the CNT bundle is 97.6 kcal kg-1. On the other hand, hydrogen liquefaction is not observed in the CNT of a single type. This is because charge polarization is not developed on the single CNT as it is symmetrically deformed under the same pressure. Consequently, the hydrogen storage capacity on the CNT bundle is much higher due to liquefaction than that on the single CNT. Additionally, our results indicate that it would also be possible to liquefy H)(2) gas on a more strongly polarized CNT bundle at temperatures higher than 80 K. 2005 American Institute of Physics.This research was supported by a grant sCode No.
04K1501-02210d from “Center for Nanostructured Materials
Technology” under “21st Century Frontier R&D Programs”
of the Ministry of Science and Technology, Korea
Theoretical study on interaction of hydrogen with single-walled boron nitride nanotubes. II. Collision, storage, and adsorption
Full text linkCollision and adsorption of hydrogen with high incident kinetic energies on a single-walled boron nitride (BN) nanotube have been investigated. Molecular-dynamics (MD) simulations indicate that at incident energies below 14 eV hydrogen bounces off the BN nanotube wall. On the other hand, at incident energies between 14 and 22 eV each hydrogen molecule is dissociated at the exterior wall to form two hydrogen atoms, but only one of them goes through the wall. However, at the incident energies between 23 and 26 eV all of the hydrogen atoms dissociated at the exterior wall are found to be capable of going inside the nanotube and then to recombine to form hydrogen molecules inside the nanotube. Consequently, it is determined that hydrogen should have the incident energy > 22 eV to go inside the nanotube. On the other hand, we find that the collisions using the incident energies > 26 eV could result in damaging the nanotube structures. In addition our MD simulations find that hydrogen atoms dissociated at the wall cannot bind to either boron or nitrogen atoms in the interior wall of the nanotube. (c) 2005 American Institute of Physics.This research was supported by a grant (code No. 04K1501-02210) from “Center for Nanostructured Materials Technology” under “21st Century Frontier R&D Programs”
of the Ministry of Science and Technology, Korea
The theoretical study on interaction of hydrogen with single-walled boron nitride nanotubes. I. The reactive force field ReaxFFHBN development
Full text linkWe present a new reactive force field ReaxFF(HBN) derived to accurately model large molecular and condensed phase systems of H, B, and N atoms. ReaxFF(HBN) has been tested against quantum calculation data for B-H, B-B, and B-N bond dissociations and for H-B-H, B-N-B, and N-B-N bond angle strain energies of various molecular clusters. The accuracy of the developed ReaxFF(HBN) for B-N-H systems is also tested for (i) H-B and H-B bond energies as a function of out of plane in H-B(NH2)(3) and H-N(BH2)(3), respectively, (ii) the reaction energy for the B3N3H6+H-2-> B3N3H8, and (iii) crystal properties such as lattice parameters and equations of states for the hexagonal type (h-BN) with a graphite structure and for the cubic type (c-BN) with a zinc-blende structure. For all these systems, ReaxFF(HBN) gives reliable results consistent with those from quantum calculations as it describes well bond breaking and formation in chemical processes and physical properties. Consequently, the molecular-dynamics simulation based on ReaxFF(HBN) is expected to give a good description of large systems (> 2000 atoms even on the one-CPU machine) with hydrogen, boron, and nitrogen atoms. (c) 2005 American Institute of Physics.This research was supported by a grant (Code No.
04K1501-02210) from “Center for Nanostructured Materials
Technology” under “21st Century Frontier R&D Programs”
of the Korean Ministry of Science and Technology
Molecular dynamics investigation into the adsorption of organic compounds on kaolinite surfaces
No full textMolecular dynamics simulations have been performed to determine the partitioning behaviour of organic compounds between water phases and inorganic surfaces. In the first of these sets of simulations the heat of adsorption of a single benzocarbazole molecule on water-wet kaolinite surfaces was determined. The results from these simulations show that the benzocarbazole isomers have a slight preference for being adsorbed on the water-wet kaolinite mineral surfaces over being desorbed in the water phase (about -1.5 kcal/mol for the kaolinite alumina surfaces and about -4 kcal/mol for the kaolinite silica surface). No significant differences between the adsorption behaviour of benzo[a]- or benzo[c]carbazole were found. In a second set of simulations the stabilities of four different configurations of a three-phase water/cyclohexane/kaolinite system were determined. The results from these simulations show that a fully water-wet kaolinite is thermodynamically preferred over a fully cyclohexane-wet kaolinite system and that the silica-surface of kaolinite has a higher affinity for the water-phase than the alumina surface. The contrasting results from these two sets of simulations show that the phase behaviour of a single organic molecule in a water/mineral surface system is not necessarily related to the behaviour of an equivalent organic phase in the same system. \ua9 2001 Elsevier Science Ltd
Application of molecular dynamics calculations in the prediction of dynamical molecular properties
No full textKnowledge of dynamical molecular properties like partition coefficients and diffusion constants are of vital importance for a reliable description of the distribution of organic material in the subsurface. However, such data, and especially their temperature and pressure dependence, are sometimes difficult to obtain by experimental means under subsurface conditions. Molecular dynamics, a computational technique aiming to describe the time-dependent movement of molecules, may provide an interesting alternative method to reliably estimate dynamical properties. To test its applicability to a geochemical problem molecular dynamics simulations were used to predict the oil/water partition coefficients (K(o/w)) of several phenol and carbazole compounds. A limited molecular dynamics simulation indeed managed to properly predict the qualitative differences in oil/water partition coefficients between these compounds (K(o/w)(phenol) < K(o/w)(o- and m-cresol) < K(o/w)(carbazole) < K(o/w)(benzo[a] and benzo[c]carbazole). An extended molecular dynamics simulation was performed to predict the relative K(o/w) of benzo[a] and benzo[c]carbazole. The results from this simulation suggests that benzo[c]carbazole has a somewhat higher affinity for the hydrocarbon phase than its benzo[a]isomer, confirming measured solubilities of these compounds in hydrocarbon solvents
Unravelling Mango\u27s mysteries: A kinetic scheme describing the diagenetic fate of C7-alkanes in petroleum systems
No full textAnalyses of the composition of the light (gasoline range) hydrocarbons in oils are widely used to assess oil maturity or to track petroleum alteration. As Mango observed, the concentration ratios of some isomeric alkanes in the gasoline range do not match their relative thermodynamic stabilities, although these compounds are probably derived from the same alkene precursors. This indicates that light hydrocarbon isomerisation is proceeding via a kinetically rather than an energetically controlled reaction mechanism. We present here a reaction scheme for the formation of isomeric C(r)heptanes from heptenes and kinetic calculations based on this scheme, that result in 2-methylhexane/3-methylhexane ratios very similar to those observed in sediments. As such, these results provide a useful theoretical backbone for the utilisation of the gasoline range hydrocarbon compounds in petroleum geochemistry
A computational chemical study of penetration and displacement of water films near mineral surfaces
No full textA series of molecular dynamics simulations have been performed on organic-water mixtures near mineral surfaces. These simulations show that, in contrast to apolar compounds, small polar organic compounds such as phenols can penetrate through thin water films to adsorb on these mineral surfaces. Furthermore, additional simulations involving demixing of an organic-water mixture near a surfactant-covered mineral surface demonstrate that even low concentrations of adsorbed polar compounds can induce major changes in mineral surface wettability, allowing sorption of apolar molecules. This strongly supports a two-stage adsorption mechanism for organic solutes, involving initial migration of small polar organic molecules to the mineral surface followed by water film displacement due to co-adsorption of the more apolar organic compounds, thus converting an initial water-wet mineral system to an organic-covered surface. This has profound implications for studies of petroleum reservoir diagenesis and wettability changes. \ua9 The Royal Society of Chemistry and the Division of Geochemistry of the American Chemical Society 2000
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