Effect of varying the 1–4 intramolecular scaling factor in atomistic simulations of long-chain N-alkanes with the OPLS-AA model

Ye, Xianggui; Cui, Shaogang; Almeida, Valmor F. de; Khomami, Bamin

doi:10.1007/s00894-012-1651-5

Cited by 33 publications

(40 citation statements)

References 28 publications

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“…The initial parameterizations of the popular all-atom force-fields OPLS-AA and AMBER-AA over-predicted the density by 14% and 11%, respectively, at ambient conditions. This density over-prediction has been observed in previous simulations of long chain alkanes using these force-fields at ambient conditions and has been attributed to crystallization due to the over-prediction of the melting point (Figure 1a) [16,17,20]. Crystallization did not occur at 423 K and 60.8 MPa, and both OPLS-AA and AMBER-AA under-predicted the density by around 4%.…”

Section: Resultssupporting

confidence: 61%

“…Although improvements to the prediction of transport properties by united-atom force-fields have been made by using modified forms of non-bonded interaction (other than Lennard-Jones), such changes can often have a detrimental effect on the thermodynamic properties for which the force-fields were originally parameterized [18]. It has also been shown that many popular all-atom force-fields yield a much higher melting point for long-chain alkanes than the experimental value, which in turn leads to drastically elevated density and viscosity values [16,17,19,20]. This may be critical in simulations of confined systems, where intricate phase transitions can heavily influence the tribological behavior observed [4,21,22].…”

Section: Introductionmentioning

confidence: 99%

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A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

et al. 2016

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For the successful development and application of lubricants, a full understanding of their complex nanoscale behavior under a wide range of external conditions is required, but this is difficult to obtain experimentally. Nonequilibrium molecular dynamics (NEMD) simulations can be used to yield unique insights into the atomic-scale structure and friction of lubricants and additives; however, the accuracy of the results depend on the chosen force-field. In this study, we demonstrate that the use of an accurate, all-atom force-field is critical in order to; (i) accurately predict important properties of long-chain, linear molecules; and (ii) reproduce experimental friction behavior of multi-component tribological systems. In particular, we focus on n-hexadecane, an important model lubricant with a wide range of industrial applications. Moreover, simulating conditions common in tribological systems, i.e., high temperatures and pressures (HTHP), allows the limits of the selected force-fields to be tested. In the first section, a large number of united-atom and all-atom force-fields are benchmarked in terms of their density and viscosity prediction accuracy of n-hexadecane using equilibrium molecular dynamics (EMD) simulations at ambient and HTHP conditions. Whilst united-atom force-fields accurately reproduce experimental density, the viscosity is significantly under-predicted compared to all-atom force-fields and experiments. Moreover, some all-atom force-fields yield elevated melting points, leading to significant overestimation of both the density and viscosity. In the second section, the most accurate united-atom and all-atom force-field are compared in confined NEMD simulations which probe the structure and friction of stearic acid adsorbed on iron oxide and separated by a thin layer of n-hexadecane. The united-atom force-field provides an accurate representation of the structure of the confined stearic acid film; however, friction coefficients are consistently under-predicted and the friction-coverage and friction-velocity behavior deviates from that observed using all-atom force-fields and experimentally. This has important implications regarding force-field selection for NEMD simulations of systems containing long-chain, linear molecules; specifically, it is recommended that accurate all-atom potentials, such as L-OPLS-AA, are employed.

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Section: Resultssupporting

confidence: 61%

Section: Introductionmentioning

confidence: 99%

A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

et al. 2016

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“…AA force-fields are much more computationally expensive and, as originally developed, many of them resulted in anomalous solid−liquid phase behaviour for the long-chain alkanes relevant to tribology [80]. Fortunately, the latter issue has been overcome by re-parameterising AA force-fields specifically for long-chain alkanes [8,9], which has led to far more accurate density and viscosity prediction (≈ 10% for n-hexadecane [10]), even under high temperature and high pressure (HTHP) conditions [10].…”

Section: Classical Force-fieldsmentioning

confidence: 99%

“…Fortunately, the latter issue has been overcome by re-parameterising AA force-fields specifically for long-chain alkanes [8,9], which has led to far more accurate density and viscosity prediction (≈ 10% for n-hexadecane [10]), even under high temperature and high pressure (HTHP) conditions [10]. More modest improvements in the density prediction of lubricantsized alkanes using AA force-fields can be made simply by reducing the intramolecular 1-4 scaling parameters for the L−J and electrostatic interactions using otherwise unmodified force-fields [80]. An increasing number of tribological NEMD studies are now employing accurate AA force-fields in order to yield more realistic viscous behaviour; however, for some types of molecules (multiple small branches, e.g., squalane) the additional computational expense compared to UA force-fields may not be justified.…”

Section: Classical Force-fieldsmentioning

confidence: 99%

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

2018

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Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.

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“…15 For n-dodecane, we used a re-optimized OPLS All-Atom force field with scaling factor at 0.4. 18 The parameters for cross interactions between different molecular species (atomic groups) were derived using the standard Lorentz-Berthelot rules. 19 The particle−particle particle−mesh method, 20 as implemented in LAMMPS, 16 with a real space truncation distance of 15 Å and an accuracy of 10 -6 was used to treat the electrostatic interactions.…”

Section: Molecular Modelsmentioning

confidence: 99%

Quantify Water Extraction by TBP/Dodecane via Molecular Dynamics Simulations

Khomami

Cui

Almeida

et al. 2013

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Objective: The purpose of this project is to quantify the interfacial transport of water into the most prevalent nuclear reprocessing solvent extractant mixture, namely tri-butylphosphate (TBP) and dodecane, via massively parallel molecular dynamics simulations on the most powerful machines available for open research. Specifically, we will accomplish this objective by evolving the water/TBP/dodecane system up to 1 ms elapsed time, and validate the simulation results by direct comparison with experimentally measured water solubility in the organic phase. The significance of this effort is to demonstrate for the first time that the combination of emerging simulation tools and state-of-the-art supercomputers can provide quantitative information on par to experimental measurements for solvent extraction systems of relevance to the nuclear fuel cycle.Efforts and Results: To achieve the object described above, initially, the single component, single phase systems were studied to verify existing and develop new molecular models, followed by a single phase mixture, and then finally the complete twophase water extraction system. Specifically, the systems we studied are: (1) pure TB; (2) n-doedecane; (3) TBP/n-dodecane mixture; and (4) the complete extraction system: water-TBP/n-dodecane two phase system to gain deep insight into the water extraction process. We have completely achieved our goal of extracting water molecules into the TBP/n-dodecane mixture to saturation and obtained favorable comparison with experiment. Many insights into fundamental molecular level processes and physics were obtained from the process. Most importantly, we found that the dipole moment of the 2 Bu=C 4 extracting agent is crucially important in affecting the interface roughness and the extraction rate of water molecules into the organic phase. In addition, we have identified shortcomings in the existing OPLS-AA force field potential for long-chain alkanes. The significance of this force field is that it is supposed to be optimized for molecular liquid simulations. We found that it failed for n-dodecane and/or longer chains to correctly predict the liquid state and thus not usable for our water extraction simulation. We have proposed a simple modification to circumvent the artificial crystallization of the long alkane chains applicable at ambient condition.(b) Effort performed and the accomplishments achieved TBP Force Field Model DevelopmentWe conducted force field survey on 4 sets of force field parameters for TBP, a molecule composed of a phosphate head group and three butyl tails as shown in Figure 1. The 4 sets of parameters are based on adoption of Van der Waals parameters for the phosphoryl head group and the butyl tail group of the TBP molecules, from separate force field models already in use. In particular, we used the Amber parameters for the Van der Waals interaction of the phosphoryl group, combined with either the Amber or the OPLS-AA (Optimized Potential for Liiquid Simulation-All Atoms) force field for the butyl tail...

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Effect of varying the 1–4 intramolecular scaling factor in atomistic simulations of long-chain N-alkanes with the OPLS-AA model

Cited by 33 publications

References 28 publications

A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

Quantify Water Extraction by TBP/Dodecane via Molecular Dynamics Simulations

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