Molecular dynamics simulations of laser-induced incandescence of soot using an extended ReaxFF reactive force field

Laser-induced incandescence (LII) of soot has developed into a popular method for making in situ measurements of soot volume fraction and primary particle sizes. However, there is still a lack of understanding regarding the generation and interpretation of the cooling signals. To model heat transfer from the heated soot particles to the surrounding gas, knowledge of the collision-based cooling as well as reactive events, including oxidation (exothermic) and evaporation (endothermic) is essential. We have simulated LII of soot using the ReaxFF reactive force field for hydrocarbon combustion. Soot was modeled as a stack of four graphene sheets linked together using sp(3) hybridized carbon atoms. To calculate the thermal accommodation coefficient of various gases with soot, graphene sheets of diameter 40 ? were used to create a soot particle containing 2691 atoms, and these simulations were carried out using the ReaxFF version incorporated into the Amsterdam Density Functional program. The reactive force field enables us to simulate the effects of conduction, evaporation, and oxidation of the soot particle on the cooling signal. Simulations were carried out for both reactive and nonreactive gas species at various pressures, and the subsequent cooling signals of soot were compared and analyzed. To correctly model N(2)-soot interactions, optimization of N-N and N-C-H force field parameters against DFT and experimental values was performed and is described in this paper. Subsequently, simulations were performed in order to find the thermal accommodation coefficients of soot with various monatomic and polyatomic gas molecules like He, Ne, Ar, N(2), CO(2), and CH(4). For all these species we find good agreement between our ReaxFF results and previously published accommodation coefficients. We thus believe that Molecular Dynamics using the ReaxFF reactive force field is a promising approach to simulate the physical and chemical aspects of soot LII.     

First principle and ReaxFF molecular dynamics investigations of formaldehyde dissociation on Fe(100) surface

Detailed formaldehyde adsorption and dissociation reactions on Fe(100) surface were studied using first principle calculations and molecular dynamics (MD) simulations, and results were compared with available experimental data. The study includes formaldehyde, formyl radical (HCO), and CO adsorption and dissociation energy calculations on the surface, adsorbate vibrational frequency calculations, density of states analysis of clean and adsorbed surfaces, complete potential energy diagram construction from formaldehyde to atomic carbon (C), hydrogen (H), and oxygen (O), simulation of formaldehyde adsorption and dissociation reaction on the surface using reactive force field, ReaxFF MD, and reaction rate calculations of adsorbates using transition state theory (TST). Formaldehyde and HCO were adsorbed most strongly at the hollow (fourfold) site. Adsorption energies ranged from ?22.9 to ?33.9 kcal/mol for formaldehyde, and from ?44.3 to ?66.3 kcal/mol for HCO, depending on adsorption sites and molecular direction. The dissociation energies were investigated for the dissociation paths: formaldehyde → HCO + H, HCO → H + CO, and CO → C + O, and the calculated energies were 11.0, 4.1, and 26.3 kcal/mol, respectively. ReaxFF MD simulation results were compared with experimental surface analysis using high resolution electron energy loss spectrometry (HREELS) and TST based reaction rates. ReaxFF simulation showed less reactivity than HREELS observation at 310 and 523 K. ReaxFF simulation showed more reactivity than the TST based rate for formaldehyde dissociation and less reactivity than TST based rate for HCO dissociation at 523 K. TST‐based rates are consistent with HREELS observation. © 2013 Wiley Periodicals, Inc.     

Comparing the mechanical response of di‐, tri‐, and tetra‐functional resin epoxies with reactive molecular dynamics

The influence of monomer functionality on the mechanical properties of epoxies is studied using molecular dynamics (MD) with the Reax Force Field (ReaxFF). From deformation simulations, the Young's modulus, yield point, and Poisson's ratio are calculated and analyzed. Comparison between the network structures of distinct epoxies is further advanced by the monomeric degree index (MDI). Experimental validation demonstrates the MD results correctly predict the relationship in Young's moduli. Therefore, ReaxFF is confirmed to be a useful tool for studying the mechanical behavior of epoxies. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 255–264     

Optimization and application of lithium parameters for the reactive force field, ReaxFF

To 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, Li(2)H(2), H(3)C-Li, H(3)C-H(2)C-Li, H(2)C=C-LiH, HCCLi, H(6)C(5)-Li, and Li(2)C(2), 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.     

Effects of fuel additives on the thermal cracking of n-decane from reactive molecular dynamics

Thermal cracking of n-decane and n-decane in the presence of several fuel additives are studied in order to improve the rate of thermal cracking by using reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field. From MD simulations, we find the initiation mechanisms of pyrolysis of n-decane are mainly through two pathways: (1) the cleavage of a C-C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding decyl radical. Another pathway is the H-abstraction reactions by small radicals including H, CH(3), and C(2)H(5). The basic reaction mechanisms are in good agreement with existing chemical kinetic models of thermal decomposition of n-decane. Quantum mechanical calculations of reaction enthalpies demonstrate that the H-abstraction channel is easier compared with the direct C-C or C-H bond-breaking in n-decane. The thermal cracking of n-decane with several additives is further investigated. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis. The different chemical structures of the fuel additives greatly affect the apparent activation energy and pre-exponential factors. The presence of diethyl ether (DEE), methyl tert-butyl ether (MTBE), 1-nitropropane (NP), 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexaoxonane (TEMPO), triethylamine (TEA), and diacetonediperodixe (DADP) exhibit remarkable promoting effect on the thermal cracking rates, compared with that of pure n-decane, in the following order: NP > TEMPO > DADP > DEE (～MTBE) > TEA, which coincides with experimental results. These results demonstrate that reactive MD simulations can be used to screen for fuel additives and provide useful information for more comprehensive chemical kinetic model studies at the molecular level.     

Development and application of a ReaxFF reactive force field for hydrogen combustion

To investigate the reaction kinetics of hydrogen combustion at high-pressure and high-temperature conditions, we constructed a ReaxFF training set to include reaction energies and transition states relevant to hydrogen combustion and optimized the ReaxFF force field parameters against training data obtained from quantum mechanical calculations and experimental values. The optimized ReaxFF potential functions were used to run NVT MD (i.e., molecular dynamics simulation with fixed number of atoms, volume, and temperature) simulations for various H(2)/O(2) mixtures. We observed that the hydroperoxyl (HO(2)) radical plays a key role in the reaction kinetics at our input conditions (T ≥ 3000 K, P > 400 atm). The reaction mechanism observed is in good agreement with predictions of existing continuum-scale kinetic models for hydrogen combustion, and a transition of reaction mechanism is observed as we move from high pressure, low temperature to low pressure, high temperature. Since ReaxFF derives its parameters from quantum mechanical data and can simulate reaction pathways without any preconditioning, we believe that atomistic simulations through ReaxFF could be a useful tool in enhancing existing continuum-scale kinetic models for prediction of hydrogen combustion kinetics at high-pressure and high-temperature conditions, which otherwise is difficult to attain through experiments.     

Characterization of the active site of yeast RNA polymerase II by DFT and ReaxFF calculations

Most known DNA-dependent RNA polymerases (RNAPs) share a universal heptapeptide, called the NADFDGD motif. The crystal structures of RNAPs indicate that in all cases this motif forms a loop with an embedded triad of aspartic acid residues. This conserved loop is the key part of the active site. Based on the crystal structures of the yeast RNAP II, we have studied this common active site for three cases: (1) single RNAP, (2) pre-translocation elongation complex, and (3) post-translocation elongation complex. Here we have applied two different modeling methods, the GGA density functional theory method (PBE) of quantum mechanics (QM) and the ReaxFF reactive force field. The QM calculations indicate that the loop shrinks from pre- to post-translocation and expands from post- to pre- translocation. In addition, PBE MD simulations in the gas phase at 310 K shows that the loop in the single-RNAP case is tightly connected to a catalytic Mg ²⁺ ion and that there is an ordered hydrogen bond network in the loop. The corresponding ReaxFF MD simulation presents a less stable loop structure, suggesting that ReaxFF may underestimate the coordinating interactions between carbonyl oxygen and magnesium ion compared to the gas phase QM. However, with ReaxFF it was practical to study the dynamics for a much more detailed model for the post-translocational case, including the complete loop and solvent. This leads to a plausible reactant-side model that may explain the large difference in efficiency of NTP polymerization between RNA and DNA polymerases.     

Development and validation of COMPASS force field parameters for molecules with aliphatic azide chains

To establish force-field-based (molecular) modeling capability that will accurately predict condensed-phase thermophysical properties for materials containing aliphatic azide chains, potential parameters for atom types unique to such chains have been developed and added to the COMPASS force field. The development effort identified the need to define four new atom types: one for each of the three azide nitrogen atoms and one for the carbon atom bonded to the azide. Calculations performed with the expanded force field yield (gas-phase) molecular structures and vibrational frequencies for hydrazoic acid, azidomethane, and the anti and gauche forms of azidoethane in good agreement with values determined experimentally and/or through computational quantum mechanics. Liquid densities calculated via molecular dynamics (MD) simulations were also in good agreement with published values for 13 of 15 training set compounds, the exceptions being hydrazoic acid and azidomethane. Of the 13 compounds whose densities are well simulated, nine have experimentally determined heats of vaporization reported in the open literature, and in all of these cases, MD simulated values for this property are in reasonable agreement with the published values. Simulations with the force field also yielded reasonable density estimates for a series of 2-azidoethanamines that have been synthesized and tested for use as hydrazine-alternative fuels.     

Simulations on the thermal decomposition of a poly(dimethylsiloxane) polymer using the ReaxFF reactive force field

To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon-silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH(3) radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si-C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH(3) radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH(3) radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO(2) slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C(2) hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers.     

Atomistic-scale simulations of the initial chemical events in the thermal initiation of triacetonetriperoxide

To study the initial chemical events related to the detonation of triacetonetriperoxide (TATP), we have performed a series of molecular dynamics (MD) simulations. In these simulations we used the ReaxFF reactive force field, which we have extended to reproduce the quantum mechanics (QM)-derived relative energies of the reactants, products, intermediates, and transition states related to the TATP unimolecular decomposition. We find excellent agreement between the QM-predicted reaction products and those observed from 100 independent ReaxFF unimolecular MD cookoff simulations. Furthermore, the primary reaction products and average initiation temperature observed in these 100 independent unimolecular cookoff simulations match closely with those observed from a TATP condensed-phase cookoff simulation, indicating that unimolecular decomposition dominates the thermal initiation of the TATP condensed phase. Our simulations demonstrate that thermal initiation of condensed-phase TATP is entropy-driven (rather than enthalpy-driven), since the initial reaction (which mainly leads to the formation of acetone, O(2), and several unstable C(3)H(6)O(2) isomers) is almost energy-neutral. The O(2) generated in the initiation steps is subsequently utilized in exothermic secondary reactions, leading finally to formation of water and a wide range of small hydrocarbons, acids, aldehydes, ketones, ethers, and alcohols.     

A detailed reaction mechanism for hexamethyldisiloxane combustion via experiments and ReaxFF molecular dynamics simulations

Hexamethyldisiloxane (HMDSO) is one of the main impurities in the syngas produced from sewage and landfill plants. In order to utilize this syngas or control the characteristics of the generated silica particles, it is crucial to understand the chemical kinetics of HMDSO combustion. This study investigated the process of HMDSO combustion using synchrotron radiation mass spectrometry (SRMS), gas chromatography (GC), and ReaxFF molecular dynamics simulations. First, the force field used for ReaxFF simulation was validated by comparing the energies of different bond lengths, bond angles, and dihedral angles with the ones from DFT calculations. Good agreements were found. Then, ReaxFF simulations of HMDSO combustion with this force field were conducted under various conditions, which include different equivalence ratios (0.67, 1.0, and 1.5) and temperatures ranging from 2000 to 3500 K. The oxidation characteristics of HMDSO were analyzed, including the evolution of gas products and particle formation. Finally, based on the results from experiments and ReaxFF simulations, the reaction pathways, reaction lists, and reaction kinetics data during HMDSO combustion were obtained. A detailed reaction mechanism was proposed and validated by applying it in modeling the H₂/HMDSO/O₂ combustion systems. The temperature and part of the gas products such as CO and CO₂ as well as SiO could be well predicted.     

基于ReaxFF力场的对硝基苯酚臭氧氧化分子动力学模拟

本文采用基于ReaxFF反应力场的分子动力学方法(ReaxFF MD),利用自主研发的国际首个基于GPU加速的ReaxFF MD程序系统GMD-Reax和独特的化学反应分析工具VARx MD,探索臭氧氧化对硝基苯酚的反应机理。通过模拟考察了300 K恒温条件下臭氧氧化水中对硝基苯酚的行为,获得了酚结构开环、CO_2生成、主要自由基(·OH、·O_2、·O)及团簇型自由基的数量演变趋势,并可定性描述六元环开环和CO_2生成均遵循伪一级反应动力学规律。反应机理分析表明酚类分子在水溶液中被臭氧氧化的路径主要经过攫氢、六元环开环、碳链的氧化分解三个阶段,也揭示了自由基和团簇型自由基在臭氧降解对硝基苯酚时所发挥的重要作用。本工作是应用ReaxFF MD分子模拟方法对常温水环境下臭氧降解酚类污染物反应机理研究的一个尝试,可为深入认识该机理及相关的实验、理论研究提供一定的参考。     

Conformational analysis of L-prolines in water

The results of the ring conformational analysis of L-proline, N-acetyl-L-proline, and trans-4-hydroxy-L-proline by NMR combined with calculations using density functional theory (DFT) and molecular dynamics (MD) are reported. Accurate values of 1H-1H J-couplings in water and other solvents have been determined. Using a two-site equilibrium model, the Cgamma-endo conformer of L-proline in water has been identified as intermediate between gammaTdelta [twist(Cgamma-endo, Cdelta-exo)] and gammaE [envelope(Cgamma-endo)] and the Cgamma-exo conformer as betaTgamma. Both conformers were equally populated at room temperature. The N-acetyl [cis-rotamer gammaTbeta(80%)/gammaE(20%) and trans-rotamer gammaTbeta(61%)/gammaE(39%)] and 4-hydroxy (gammaEpsilon) derivatives showed significant changes in both the population and the geometries of the preferred ring conformers. The combination of NMR predicted populations with the DFT B3LYP/6-311+G(2d,p)/IEFPCM calculations proved successful, resulting in fairly accurate predictions of J-couplings. Simulations using MD were mostly in favor of the two-site equilibrium model between Cgamma-endo and Cgamma-exo conformers, similar to that used for the analysis of NMR J-couplings. Various force fields examined for MD simulations failed to reproduce the ring conformational geometries and populations of L-proline in water accurately, while significantly better agreement with NMR was found for trans-N-acetyl-L-proline using GROMOS and AMBER force fields.     

FMO‐MD Simulations on the Hydration of Formaldehyde in Water Solution with Constraint Dynamics

Full‐quantum mechanical fragment molecular orbital‐based molecular dynamics (FMO‐MD) simulations were applied to the hydration reaction of formaldehyde in water solution under neutral conditions. Two mechanisms, a concerted and a stepwise one, were considered with respect to the nucleophilic addition and the proton transfer. Preliminary molecular orbital calculations by means of polarized continuum model reaction field predicted that the hydration prefers a concerted mechanism. Because the calculated activation barriers were too high for free FMO‐MD simulations to give reactive trajectories spontaneously, a More O’Ferrall–Jencks‐type diagram was constructed from the statistical analysis of the FMO‐MD simulations with constraint dynamics. The diagram showed that the hydration proceeds through a zwitterionic‐like (ZW‐like) structure. The free energy changes along the reaction coordinate calculated by means of the blue moon ensemble for the hydration and the amination of formaldehyde indicated that the hydration proceeds through a concerted process through the ZW‐like structure, whereas the amination goes through a stepwise mechanism with a ZW intermediate. In inspection of the FMO‐MD trajectories, water‐mediated cyclic proton transfers were observed in both reactions on the way from the ZW‐like structure to the product. These proton transfers also have an asynchronous character, in which deprotonation from the nucleophilic oxygen atom (or nitrogen atom for amination) precedes the protonation of the carbonyl oxygen atom. The results showed the strong advantage of the FMO‐MD simulations to obtain detailed information at a molecular level for solution reactions.     

ReaxFF(MgH) reactive force field for magnesium hydride systems

We have developed a reactive force field (ReaxFF(MgH)) for magnesium and magnesium hydride systems. The parameters for this force field were derived from fitting to quantum chemical (QM) data on magnesium clusters and on the equations of states for condensed phases of magnesium metal and magnesium hydride crystal. The force field reproduces the QM-derived cell parameters, density, and the equations of state for various pure Mg and MgH(2) crystal phases as well as and bond dissociation, angle bending, charge distribution, and reaction energy data for small magnesium hydride clusters. To demonstrate one application of ReaxFF(MgH), we have carried out MD simulations on the hydrogen absorption/desorption process in magnesium hydrides, focusing particularly on the size effect of MgH(2) nanoparticles on H(2) desorption kinetics. Our results show a clear relationship between grain size and heat of formation of MgH(2); as the particle size decreases, the heat of formation increases. Between 0.6 and 2.0 nm, the heat of formation ranges from -16 to -19 kcal/Mg and diverges toward that of the bulk value (-20.00 kcal/Mg) as the particle diameter increases beyond 2 nm. Therefore, it is not surprising to find that Mg nanoparticles formed by ball milling (20-100 nm) do not exhibit any significant change in thermochemical properties.     

Binding energies of five molecular pincers calculated by explicit and implicit solvent models

Molecular pincers or tweezers are designed to hold and release the target molecule. Potential applications involve drug distribution in medicine, environment technologies, or microindustrial techniques. Typically, the binding is dominated by van der Waals forces. Modeling of such complexes can significantly enhance their design; yet obtaining accurate complexation energies by theory is difficult. In this study, density functional theory (DFT) computations combined with dielectric continuum solvent model are compared with the potential of mean force approach using umbrella sampling and the weighted histogram analysis method (WHAM) with molecular dynamics (MD) simulations. For DFT, functional and basis set effects are discussed. The computed results are compared to experimental data based on NMR spectroscopic measurements of five synthesized tweezers based on the Tröger's basis. Whereas the DFT computations correctly provided the observed trends in complex stability, they failed to produce realistic magnitudes of complexation energies. Typically, the binding was overestimated by DFT if compared to experiment. The simpler semiempirical PM6‐DH2X scheme proposed lately yielded better magnitudes of the binding energies than DFT but not the right order. The MD‐WHAM simulations provided the most realistic Gibbs binding energies, although the approximate MD force fields were not able to reproduce completely the ordering of relative stabilities of model complexes found by NMR. Yet the modeling provides interesting insight into the complex geometry and flexibility and appears as a useful tool in the tweezers' design. © 2012 Wiley Periodicals, Inc.     

Experimental and theoretical evaluation of synthetized cobalt oxide for phenol adsorption: Adsorption isotherms,kinetics, and thermodynamic studies

Water pollution by phenolic composites is considered a major environmental problem. Therefore, their removal by adsorption is of great practical importance. In this paper, the synthesized cobalt oxide Co₃O₄ was used as an adsorbent for the adsorption of phenol in an aqueous medium. A DFT calculation has been carried out to determine the sites accountable for the interactions in phenol molecule, and molecular dynamics (MD) simulations were used to understand the mechanism of interaction between phenol molecule and Co₃O₄ surface. The developed adsorbent was characterized by physicochemical methods including XRD, SEM, FT-IR, and BET. The maximum adsorption capacity was observed at pH = 4 with an adsorbed amount of 8.10 mg/g and (R = 98 %). Furthermore, to probe the adsorption action of the phenolic emulsion on the cobalt oxide face, theoretical simulations based on MD (molecular dynamics) and DFT (viscosity functional proposal) were performed. The DFT results verified the chemisorption ascendancy while the MD simulations indicated an increased trade of Co₃O₄ with phenol in the presence of detergent due to water-bridged H- bonds.     

Theoretical Insights into Chirality‐Controlled SWCNT Growth from a Cycloparaphenylene Template

A self‐assembly mechanism for low‐temperature SWCNT growth from a [6]cycloparaphenylene ([6]CPP) precursor via ethynyl (C₂H) radical addition is presented, based on non‐equilibrium quantum chemical molecular dynamics (QM/MD) simulations and density functional theory (DFT) calculations. This mechanism, which maintains the (6,6) armchair chirality of a SWCNT fragment throughout the growth process, is energetically more favorable than a previously proposed Diels–Alder‐based growth mechanisms [E. H. Fort, et al., J. Mater. Chem. 2011 , 21, 1373]. QM/MD simulations and DFT calculations show that C₂H radicals play dual roles during SWCNT growth, by abstracting hydrogen from the SWCNT fragment and providing the carbon source necessary for growth itself. Simulations demonstrate that chirality‐controlled SWCNT growth from macrocyclic hydrocarbon seed molecules with pre‐selected edge structure can be accomplished when the reaction conditions are carefully selected for hydrogen abstraction by radical species during the growth process.     

Comparison of Secondary Structure Formation Using 10 Different Force Fields in Microsecond Molecular Dynamics Simulations

We have compared molecular dynamics (MD) simulations of a β-hairpin forming peptide derived from the protein Nrf2 with 10 biomolecular force fields using trajectories of at least 1 μs. The total simulation time was 37.2 μs. Previous studies have shown that different force fields, water models, simulation methods, and parameters can affect simulation outcomes. The MD simulations were done in explicit solvent with a 16-mer Nrf2 β-hairpin forming peptide using Amber ff99SB-ILDN, Amber ff99SB*-ILDN, Amber ff99SB, Amber ff99SB*, Amber ff03, Amber ff03*, GROMOS96 43a1p, GROMOS96 53a6, CHARMM27, and OPLS-AA/L force fields. The effects of charge-groups, terminal capping, and phosphorylation on the peptide folding were also examined. Despite using identical starting structures and simulation parameters, we observed clear differences among the various force fields and even between replicates using the same force field. Our simulations show that the uncapped peptide folds into a native-like β-hairpin structure at 310 K when Amber ff99SB-ILDN, Amber ff99SB*-ILDN, Amber ff99SB, Amber ff99SB*, Amber ff03, Amber ff03*, GROMOS96 43a1p, or GROMOS96 53a6 were used. The CHARMM27 simulations were able to form native hairpins in some of the elevated temperature simulations, while the OPLS-AA/L simulations did not yield native hairpin structures at any temperatures tested. Simulations that used charge-groups or peptide capping groups were not largely different from their uncapped counterparts with single atom charge-groups. On the other hand, phosphorylation of the threonine residue located at the β-turn significantly affected the hairpin formation. To our knowledge, this is the first study comparing such a large set of force fields with respect to β-hairpin folding. Such a comprehensive comparison will offer useful guidance to others conducting similar types of simulations.