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We have parametrized an AMBER force field in a TEAMFF database to accurately represent intra- and intermolecular interactions of cellulose. Parameters are obtained by fitting quantum mechanics (QM) energetic data on a training set of 12 simple and substituted heterocycles, alcohols, ethers, and saccharides. The temperature-dependent Lennard–Jones parameters were optimized by fitting experimental data of 23 molecular liquids at different temperatures using molecular dynamics (MD) simulations. Validation on cellobiose, the monomer of cellulose, exhibits excellent agreement between the data obtained by molecular mechanics calculations using the present force field and QM calculations in terms of conformational energies, structures, and vibrational frequencies. The MD simulation results of cellulose Iβ crystal agree well with the experimental data, showing a 0.1% deviation in density and less than 1% deviations in all the cell parameters.

The pressure-dependent viscosity of 2,2,4-trimethylhexane is predicted using all-atom TEAM force field and the non-equilibrium periodic perturbation (PP) method. The viscosities predicted using the PP method do not show Newtonian plateau in the precision range of prediction for high-viscosity fluids. Using four alkane molecules as test cases, we found that our previously published force field, developed for ambient pressure conditions, is inaccurate for predicting viscosities under elevated pressures. Two approaches were attempted to enhance the pressure transferability of the force field: one involves moving the non-bond interaction site of hydrogen closer to that of attached carbon, which smooths the van der Waals (vdW) surface of molecule, and the other involves replacing the LJ-12-6 function by LJ-9-6, making the repulsion softer. Both approaches enhanced the pressure transferability; however, the former works well below 500 MPa, while the latter is valid under pressures up to 1 GPa.

We developed a transferable coarse-grained (CG) force field to cover polyethers and polyether blends in this work. On the basis of the perturbation theory of liquids and high-temperature expansion, we examined the theoretical foundation of using a temperature-dependent function to represent the nonbonded interactions. Using a mapping rule that balances molecular representation with the degree of coarse-graining, we defined seven CG beads that represent common polyethers. A hybrid parametrization workflow combining bottom-up and top-down approaches was applied to derive the force field parameters. The large ratio between the number of training data and the number of adjustable parameters enables the resulting force field to accurately predict many polymer properties, including cohesive energies, structural parameters, glass-transition temperatures, and surface tensions. In addition, the force field is capable of simulating polyether blends using modified Lorentz–Berthelot combination rules. As a preliminar...

Knowledge of the thermodynamic properties of molecules is essential for chemical process design and the development of new materials. Experimental measurements are often expensive and not environmentally friendly. In the past, studies using molecular simulations have focused on a specific class of molecules, owing to the lack of a consistent force field and simulation protocol. To solve this problem, we have developed a high-throughput force field simulation (HT-FFS) procedure by combining a recently developed general force field with a validated simulation protocol to calculate thermodynamic properties for large number of molecules. This procedure is applied to calculate liquid densities, heats of vaporization, heat capacities, vapor-liquid equilibrium curves, critical temperatures, critical densities and surface tensions for a wide range of alkanes. The predictions agree well with available experimental data in terms of accuracy and precision, demonstrating that HT-FFS is a valid approach to supplementing experimental measurements. Furthermore, the large amount of data generated by HT-FFS lays a foundation for machine learning. We have developed an artificial neural network that demonstrates the feasibility of expanding predictions beyond simulation using a machine learning model.

An automatic high-throughput computing (HTC) procedure is implemented for calculating vapor-liquid equilibrium (VLE) curves of binary systems using molecular dynamics simulation and coarse-grained force field. The HTC procedure builds simulation models, carries out the simulations, and validates the simulations automatically. The procedure is demonstrated by calculating the VLE curves for 16 binary mixtures of carbon dioxide and n-alkanes on 856 state points in the temperature range of 277.2–420.0 K and pressure range of 0.3–25.0 MPa. The averaged uncertainties in predictions are 0.119 MPa for pressure, 0.007 and 0.029 for liquid and vapor CO 2 molar fractions, and 0.005 and 0.010 for liquid and vapor densities respectively. The Validation against experiment data on 10 binary systems and 316 state points shows that the predictions are accurate with average deviation of about 5% for CO 2 mole fraction and 3.5% for saturated density at pressure range to P P c . This automatic procedure can only be used for the prediction of the vapor-liquid equilibrium. The computed data can be obtained from http://sun.sjtu.edu.cn/msd .

Calcium and magnesium ions play important roles in many physicochemical processes. To facilitate the investigation of phenomena related to these ions that occur over large length and time scales, a coarse-grained force field (CGFF) is developed for MgCl2 and CaCl2 aqueous solutions. The ions are modeled by CG beads with characteristics of hydration shells. To accurately describe the nonideal behavior of the solutions, osmotic coefficients in a wide range of concentrations were used as guidance for parametrization. The osmotic coefficients were obtained from the chemical potential increments of water calculated using the Bennett acceptance ratio (BAR) method. The result CGFF accurately reproduces experimental osmotic coefficients, densities, surface tensions, and cation–anion separations of calcium chloride and magnesium chloride solutions at molalities up to 3.0 mol/kg. As a preliminary application, the force field is applied to simulate aggregations of sodium dodecyl sulfate (SDS) in calcium chloride sol...

Molecular dynamics simulation is performed to compute densities and solubility parameters of polymers. TEAM force field is validated on monomers and then employed for predictions for polymers built...

The adsorption isotherm of n-perfluorohexane on BAM-P109 activated carbon was predicted using an ab initio force field and a slit-like pore model representing the activated carbon. The force field parameters were derived from data calculated using quantum mechanics density functional theory with dispersion energy correction, and the total adsorption amount was calculated as the sum of the adsorption amounts of microscopic and mesoscopic pores. It was found that the Grand Canonical Monte Carlo method can efficiently calculate adsorption on microscopic pores but not on mesoscopic pores. Thus, we divided the mesoscopic pores into surface and free-volume regions and calculated the density and thickness of the adsorbed layers and the density of the vapor phase using canonical ensemble simulations. Using this protocol, we can calculate the adsorption isotherms in good agreement with the experimental data.

The accuracy of force fields is a key to the successful prediction of the thermodynamic properties of materials. In simulations of organic molecules over large temperature ranges, atomistic force fields that are parametrized at, or near, ambient temperatures are found to systematically underestimate the intermolecular dispersion interactions at elevated temperatures. Analysis of the underestimates using diatomic molecules indicates that a minor part is due to the change in molecular polarizability, while the major part is due to the reduced dielectric constant of the bulk liquid as the density decreases with increasing temperature. By establishing the dispersion parameter as a linear function of temperature, we have successfully enhanced the temperature transferability of atomistic force fields. This approach is tested on 66 molecular liquids covering four functional groups - alkane, aromatic, ether, and ketone-aldehyde - over a broad range of temperatures by calculating liquid density, heat of vaporization, isobaric heat capacity, and shear viscosity.

The surface-bulk partition of nonionic surfactants was predicted by calculating chemical potential differences using two simulation boxes representing the surface and the bulk phases separately. A published coarse grained force field was modified and validated for this application. Thermodynamic integration (TI) was applied to compute excess chemical potentials. The high concentration surface was stabilized by applying an external harmonic potential, and the bulk was treated as ideal solution, which was confirmed by simulation using a lattice model at conditions near the critical micelle concentration (∼10(-5) mol/L). Based on the calculated chemical potential differences with precision of ca. 1 kJ/mol, the equilibria of surface-bulk concentration was predicted well in comparison with the experimental data.

Coarse-grained force field (CGFF) methods were applied to study the self-assembly of sodium dodecyl sulfate with fragrance additives. The CGFF parameters were parameterized and validated using experimental and all-atom simulation data. Direct molecular dynamics simulations were carried out to characterize the initial aggregation, partitioning of fragrances, and chemical potentials of the surfactant and fragrance molecules in aggregates of different sizes. The equilibrium critical micelle concentrations (CMCs) and micelle size distributions, which could not be obtained by direct simulation, were predicted using the calculated chemical potentials in combination with a thermodynamic model. The predicted partitioning of fragrances, CMCs, micelle sizes, and micelle structures agree well with previously reported experimental data. Graphical abstract Enhancement of micelle size distribution using thermodynamic model.

The surface-bulk partition of surfactants is a piece of important information for understanding the performance of surfactants. In this work, we predicted surface-bulk partitions for three ionic surfactants (sodium hexyl sulfate, sodium nonyl sulfate, sodium dodecyl sulfate) using molecular dynamics simulations and a coarse-grained force field. The simulations were performed with the bilayer model that represents thin films. By setting initial configurations using chemical potentials calculated for infinitely dilute solutions, we obtained a sufficient number of independent samples for each simulation which ran at least 2 μs. Predicted surface concentrations and surface tensions are in good agreement with the experimental data. The surface adsorption isotherms, surface structures, and critical micelle concentrations (CMCs) derived from the simulated data are in reasonable agreement with the experimental data and previously calculated data. For surfactants with a short alkyl chain (SHS), stable oligomers are formed before micelles in the bulk.

Grand Canonical Monte Carlo (GCMC) simulations with ab initio force field were used to predict the adsorption isotherms of n-C6F14 in BCR-704 faujasite (calcium type) zeolite. The force field parameters were derived using the vapor–liquid equilibrium data for the adsorbate molecules, and ab initio data for the adsorbent (zeolite) with argon as a testing particle. The interactions between the adsorbate molecules and the zeolite were represented by using the Lorentz–Berthelot combining rule. Based on a comparison of ab initio and experimental data obtained for methane absorbed on calcium and sodium type zeolites, it was found that the exposed, non-framework calcium cations (Ca2+) must be partially hydroxylated. As the force field parameters were determined using ab initio data, we were able to estimate the percentage of hydroxylation without interference from the force field parameters by using the experimental adsorption data of argon and nitrogen. The hydroxylation ratio of calcium cations was estimated to be 63%. Based on this model and the combined force field parameters, the adsorption isotherm of n-C6F14 on BCR-704 zeolite was predicted and the results are in reasonable agreement with the experimental data.

Using force field parameters developed and validated for zeolitic imidazolate frameworks (ZIFs) and carbon dioxide (CO2) independently from adsorption data, we predicted CO2/ZIFs adsorption isotherms using the Grand Canonic Monte Carlo (GCMC) method. The results are in sharp contradiction: the calculated adsorption data agree well with the experimental data for SOD-type ZIF-8, but are more than 100% higher than the experimental data for GME-type ZIFs. Using non-equilibrium molecular dynamics simulations and potential of mean force (PMF) calculations, we reveal that the discrepancies are due to the kinetic blockage which is significant for ZIF-68 but negligible for ZIF-8. This study demonstrates that a force field developed independently from the adsorption data can be used to predict the adsorptions accurately; and the kinetic factor must be considered if the bottlenecks exist in the adsorption paths due to geometric and energetic features of adsorbate and adsorbent. It could be very misleading if the force field parameters are adjusted by fitting the GCMC simulation data to experimental data without considering the kinetic factors.

The free energy based Lennard-Jones 12-6 (FE-12-6) coarse-grained (CG) force field developed for alkanes1 has been extended to model small molecules of light hydrocarbons (methane, ethane, propane, butane, and isobutane), nitrogen, oxygen, and carbon dioxide. The adjustable parameters of the FE-12-6 potential are determined by fitting against experimental vapor-liquid equilibrium (VLE) curves and heat of vaporization (HOV) data for pure substance liquids. Simulations using the optimized FE-12-6 parameters correctly reproduced experimental measures of the VLE, HOV, density, vapor pressure, compressibility, critical point, and surface tension for pure substances over a wide range of thermodynamic states. The force field parameters optimized for pure substances were tested on methane/butane, nitrogen/decane, and carbon dioxide/decane binary mixtures to predict their vapor-liquid equilibrium phase diagrams. It is found that for nonpolar molecules represented by different sized beads, a common scaling factor (0.08) that reduces the strength of the interaction potential between unlike beads, generated using Lorentz-Berthelot (LB) combination rules, is required to predict vapor-liquid phase equilibria accurately.

Molecular dynamics (MD) simulations were applied to investigate the liquid–liquid equilibria of water and dipropylene glycol dimethyl ether (DMM) based on all-atom OPLS force fields and simple point charge (SPC) water model. The default combining rules (geometric means) for describing interactions between water and ether molecules were modified by fitting hydration free energies of dimethyl ether and diethyl ether. The difference of Gibbs free energies (ΔG) was calculated using the thermodynamic integration (TI) method. Extensive simulations were carried out in order to reach high precision in the predicted free energies. The average uncertainties in calculated ΔG are 0.8 kJ/mol for DMM and 0.3 kJ/mol for water. From the calculated ΔGmix, the mutual solubility of the binary mixture was predicted using the double tangent method at four temperatures ranging from 283 K to 353 K and 1 atm.

Equilibrium and non-equilibrium molecular dynamics and Monte Carlo simulation techniques were applied to predict various thermodynamic, transport and vapor–liquid equilibrium properties of binary mixtures of ethylene glycol and water (EG–W) based on OPLS-AA and SPC/E force fields. The properties predicted include density, vaporization enthalpy, enthalpy of mixing, heat capacities, diffusion coefficients, shear viscosities, thermal conductivities, vapor–liquid coexistence isotherms and isobaric curves, and saturation vapor pressures. Good agreements with experimental data were obtained for most of these properties. Errors are mostly related to inaccuracy found in predictions of pure fluids; a correction to prediction of pure substance can systematically improve prediction for the mixture. This work suggests that OPLS-AA and SPC/E force fields using the common combining rules are transferable for predicting multiple physical properties of EG–W mixtures.

A classical force field capable for accurately predicting surface tensions, surface concentration, and other interfacial properties is reported for sodium dodecyl sulfate (SDS). This force field is proposed by combining parameters from well established force fields for components of the air/SDS/water interface and optimized by adjusting the van der Waals diameter of sulfate oxygen to fit experimental data of surface tension. The force field parameters are transferable; as good agreement with experimental data is achieved from independent calculations on activity derivatives of aqueous solution of sodium methyl sulfate using the Kirkwood-Buff theory. The adjusted parameter effectively modulates the electrostatic interactions of solvated ions in solutions. This modification has a strong impact to surface tension and location and mobility of sodium cations but minimal impact to properties such as density profiles of bulk phase and conformations and orientations of surfactant chain, for which consistent results compared with previous studies are obtained.

Enthalpy of mixing (EOM) is one of the most basic thermodynamic properties of mixtures. To assess feasibility of predicting EOM using force field simulation methods, fifteen (15) representative binary mixtures were investigated using MD simulations based on OPLS and TIP4P force fields. The simulation conditions and errors were carefully examined. The precision level of 0.04 kJ/mol was obtained for calculated EOM data. However, the predictions, measured by deviations from experimental data, were only qualitatively correct. Among various factors influencing the accuracy of predictions, force field quality representing interactions among different molecules plays the most significant role. Using methanol/benzene and ethanol/benzene as examples, we demonstrated that non-additive interaction terms between polarizable atoms can be used to significantly improve the quality of predictions. In addition, it appears that charge-dependent LJ parameters are required in order to represent the polarization effects accurately.

An ab initio force field that describes interactions between hydrogen molecules and IRMOF materials is proposed. The force field parameters were derived by fitting to ab initio data that includes higher-order electron correction and extended basis-set effects and validated by calculating adsorption isotherms and isosteric heats of adsorption of H2 in IRMOF-1 using GCMC simulations performed at 77 and 298 K, in a broad range of pressure from 0.0 to 8.0 MPa. Excellent agreements with experimental data were obtained. The force field was then applied to predict hydrogen-storage capacities for eight additional IRMOF materials. It was identified that the void fraction of volume (VFV) has a strong impact on the adsorption capacity, and its impacts on gravimetric and volumetric adsorption uptakes exhibit opposite trends. An overall optimal VFV is ca. 87% for IRMOFs at 77 K and 8.0 MPa.

The octanol–water partition coefficients (Ko/w) and infinite-dilution activity coefficients (γ∞) of 1-ethylpropylamine and 3-methyl-1-pentanol were predicted based on free energy calculations. The multi-configuration thermodynamic integration (TI) method was applied with molecular dynamics simulations to calculate free energy changes. With optimized simulation options and force field parameters the calculated free energy changes were highly precise (uncertainties

Temperature-programmed desorption curves for ammonia (NH3-TPD) were predicted successfully using grand canonical ensemble Monte Carlo simulation methods and force field parameters derived from quantum mechanical ab initio data. This approach provides a means to study the relationship between the structure and acidity of zeolites at the molecular level. Analysis of the predicted NH3-TPD curves reveals that zeolite pore size is a critical factor influencing the curve shape. The so-called weak and strong acids of zeolites, which give rise to two peaks in the TPD curve, are roughly related to the interactions among ammonia molecules in the pores and to the interactions between ammonia molecules and the zeolite pore walls, respectively. However, the NH3-TPD curves may still show a double peak feature without any strong acid centers if the pore is such a size that additional strain is put on the NH3 hydrogen-bond network.

An all-atom force field for ammonium perchlorate (AP) is developed with the framework of pcff force field. The structural parameters of AP obtained with the modified force field are in good agreement with experimental values. Molecular dynamics (MD) simulations have been performed to investigate AP/HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) composite at different temperatures. The binding energies, thermal expansion coefficient, and the trigger bond lengths of HMX in the AP/HMX composite have been obtained. The binding energies of the system increase slightly with temperature increasing, peak at 245 K, and then gradually decrease. The volume thermal expansion coefficient of the AP/HMX composite has been derived from the volume variation with temperature. As the temperature rises, the maximal lengths of the trigger bond N–NO2 of HMX increase gradually. The simulated results indicate that the maximal length of trigger bond can be used as a criterion for judging the sensitivity of energetic composite.

Molecular dynamics (MD) and Monte Carlo (MC) methods were applied to calculate thermodynamic properties of ethylene oxide in liquid and vapor phases. The calculations were based on the TEAM force field that was derived based on quantum mechanics data, liquid density and heat of vaporization. Without any modification, the force field was applied to predict various thermodynamic properties including vapor–liquid coexistence curves, critical points, second virial coefficient, isothermal compressibility, surface tensions, shear viscosities and thermal conductivities. Good agreements between the calculated and experimental data are obtained for most properties predicted. Although some of the deviations require further investigation, the force field appears to be transferable for predicting multiple thermodynamic properties.

Computational prediction of adsorption of small molecules in porous materials has great impact on the basic and applied research in chemical engineering and material sciences. In this work, we report an approach based on grand canonical ensemble Monte Carlo (GCMC) simulations and ab initio force fields. We calculated the adsorption curves of ammonia in ZSM-5 zeolite and hydrogen in MOF-5 (a metal-organic-framework material). The predictions agree well with experimental data. Because the predictions are based on the first principle force fields, this approach can be used for the adsorption prediction of new molecules or materials without experimental data as guidance.

The periodic perturbation method is applied to predict shear viscosities of liquid 1,4-butanediol, 1,3-butanediol, 1,2-butanediol, 2-methyl-1,3-propanediol and 1,2,4-butanetriol under two thermodynamical conditions: T = 373 K, P = 0.1 MPa and T = 373 K, P = 250 MPa. A linear dependence of the calculated shear viscosities with respect to the applied perturbation forces is identified. Based on this finding, extrapolation of the calculated viscosities to zero perturbation force is applied to estimate the shear viscosities for the “undisturbed” fluids. The uncertainties of the estimates are calculated using the block average method. The predicted values compare favorably with the experimental data. Although the force field is optimized using equilibrium properties (liquid densities and vaporization enthalpies), calculated results demonstrate that the force field can be used to predict the kinetic properties accurately. The force field parameters are well transferable among different state points. However, transferring parameters among different molecules should be executed with caution.

Isothermal–isobaric ensemble (NPT) molecular dynamics (MD) simulation was applied to calculate the heats of mixing for binary fluids based on an all-atom force field. The calculation protocol was tested on mixtures of 1-propanol/ n -heptane, n -butylamine/ n -heptane and n -butylamine/water at various temperatures and compositions. Based on a simple error analysis and comparisons of calculated and experimental data, we propose that a necessary but not sufficient condition in the predictions of the heats of mixing is to have a force field highly accurate in evaluating energetic properties for pure substances. More work is required in order to find out the sufficient conditions.

Widom insertion method coupled with canonical ensemble (NVT) molecular simulations based on different force fields was utilized to calculate Henry's law constants (HLCs) of several common gas molecules, N 2 , O 2 , CH 4 , and CO 2 , in liquid ethylene oxide and ethanol. Several simulation options, including insertion grid densities, liquid densities, force field models, and simulation methods were investigated. The results indicate that reasonably good agreements between the calculated and experimental values were obtained using the united-atom (UA) model. However, systematically overestimated values are obtained with the all-atom (AA) force field model. The causes of the problems were investigated comparatively. The data indicate that hydrogen atoms in the AA model are mainly responsible for these problems, but the underlying sampling algorithm in Widom insertions, which may be the real source of problems, needs further investigation.

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.

Ab initio molecular dynamics simulations were conducted to study the thermal decomposition mechanisms of solid FOX-7. We found that the initial decomposition of FOX-7 has three main reaction routes. The C–NO2 bond fission as reported previously is indeed the most common route. However, the inter- and intramolecular hydrogen transfers are also found to be valid, which is contradictory to previous energy minimization calculation results. The simulations agree well with known experimental data in terms of activation energy and reaction side products. A complete reaction network that describes how the main products, H2O, CO2 and N2 form in FOX-7 decomposition, is constructed from the simulation trajectories.

An all-atom force field is developed for metal organic frameworks Zn4O(RCO2)6 by fitting to quantum mechanics data. Molecular simulations are conducted to validate the force field by calculating thermal expansion coefficients, crystal bulk and Young’s moduli, power spectra, self-diffusion coefficients, and activation energies of self-diffusions for benzene and n-hexane. The calculated results are in good agreement with available experimental data. The proposed force field is suitable for simulations of adsorption or diffusion of organic molecules with flexible frameworks.

To investigate the mechanical and other condensed phase properties of energetic materials using atomistic simulation techniques, the COMPASS force field has been expanded to include high-energy nitro functional groups. In this paper, the parameterization and validation of COMPASS for nitrate esters (−ONO2) is presented. The functional forms of this force field are of the consistent force field type. The parameters were derived with an emphasis on the nonbonded parameters, which include a Lennard-Jones 9−6 function for the van der Waals (vdW) term and a Coulombic term for an electrostatic interaction. To validate the force field, molecular mechanics calculations and molecular dynamics simulations have been made on a variety of molecules containing the nitrate ester functionality. Excellent agreement has been obtained between the calculated and experimental values for molecular structures, vibrational frequencies, liquid densities, heats of vaporization, crystal structure, mechanical properties, and lattice...

A new, condensed-phase optimised ab-initio force field, COMPASS, has been developed recently. In this paper, the validation of COMPASS for phosphazenes is presented. The functional forms of this force field are of the consistent force field (CFF) type. Charges and bonded terms were derived from HF/6–31G∗ calculations, while the nonbonded parameters (L-J 9-6 vdW potential) were initially transferred from the polymer consistent force field, pcff, and optimised using MD simulations of condensed-phase properties. As a validation of COMPASS, molecular mechanics calculations and molecular dynamics simulations have been made on a number of isolated molecules, liquids, and crystals. The calculated molecular structure, vibration frequencies, conformational properties for isolated molecules, crystal cell parameters and density, liquid density, and heat of evaporation agreed favourably with most experimental data. The special conformational properties of the tetracyclophosphazenes, (NPCI2)4 and (NPF2)4, in the solid state are discussed based on molecular mechanics and CASTEP ab-initio calculations. The effect of nonbonded cutoff distance and different algorithms for pressure control in NPT simulation was also investigated. Finally, molecular dynamics using the COMPASS force field was used to predict properties of three isomers of high-molecular-weight amorphous poly(dibutoxyphosphazenes). In this case, excellent agreement was achieved between densities and glass transition temperatures obtained from dynamics and experimental data.

Various levels of ab initio calculation have been performed to determine the optimum strategy for parameterization of the valence parameters of a CFF-type force field for siloxanes and polysiloxanes. Electrostatic nonbond parameters have been determined using scaled electrostatic potential (ESP) charges adjusted for known systematic differences between ab initio and experimental data, while van der Waals nonbond parameters have been determined using a classical approach involving fitting to experimental liquid density and cohesive energy density data measured at atmospheric pressure and a single temperature for a set of four small molecules. Simulations have been performed on molecular crystals, liquids and isolated molecules, yielding results which agree favorably with available experimental data. Properties calculated include unit cell parameters and crystal densities, liquid densities from 303–473 K and 0–1800 bar, dependence of oligomer density and solubility parameters on chain length and temperature, gas-phase geometries and vibrational frequencies, and gas and liquid-phase conformational behavior.

An all-atom force field is developed for metal organic frameworks Zn₄O(RCO₂)₆ by fitting to quantum mechanics data. Molecular simulations are conducted to validate the force field by calculating thermal expansion coefficients, crystal bulk and Young's moduli, power spectra, self-diffusion coefficients, and activation energies of self-diffusions for benzene and n-hexane. The calculated results are in good agreement with available experimental data. The proposed force field is suitable for simulations of adsorption or diffusion of organic molecules with flexible frameworks.

The practical levels of density functional theory (DFT) for solids (LDA, PBE, PW91, B3LYP) are well-known not to account adequately for the London dispersion (van der Waals attraction) so important in molecular solids, leading to equilibrium volumes for molecular crystals ∼10-15% too high. The ReaxFF reactive force field is based on fitting such DFT calculations and suffers from the same problem. In the paper we extend ReaxFF by adding a London dispersion term with a form such that it has low gradients (lg) at valence distances leaving the already optimized valence interactions intact but behaves as 1/R^6 for large distances. We derive here these lg corrections to ReaxFF based on the experimental crystal structure data for graphite, polyethylene (PE), carbon dioxide, and nitrogen and for energetic materials: hexahydro-1,3,5-trinitro- 1,3,5-s-triazine (RDX), pentaerythritol tetranitrate (PETN), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and nitromethane (NM). After this dispersion correction the average error of predicted equilibrium volumes decreases from 18.5 to 4.2% for the above systems. We find that the calculated crystal structures and equation of state with ReaxFF-lg are in good agreement with experimental results. In particular, we examined the phase transition between α-RDX and γ-RDX, finding that ReaxFF-lg leads to excellent agreement for both the pressure and volume of this transition occurring at ∼4.8 GPa and ∼2.18 g/cm^3 density from ReaxFF-lg vs 3.9 GPa and ∼2.21 g/cm^3 from experiment. We expect ReaxFF-lg to improve the descriptions of the phase diagrams for other energetic materials.

We report the kinetic analysis and mechanism for the initial steps of pyrolysis and combustion of a new fuel material, 1,6-dicyclopropane-2,4-hexyne, that has enormous heats of pyrolysis and combustion, making it a potential high-energy fuel or fuel additive. These studies employ the ReaxFF force field for reactive dynamics (RD) simulations of both pyrolysis and combustion processes for both unimolecular and multimolecular systems. We find that both pyrolysis and combustion initiate from unimolecular reactions, with entropy-driven reactions being most important in both processes. Pyrolysis initiates with extrusion of an ethylene molecule from the fuel molecule and is followed quickly by isomerization of the fuel molecule, which induces additional radicals that accelerate the pyrolysis process. In the combustion process, we find three distinct mechanisms for the O(2) attack on the fuel molecule: (1) attack on the cyclopropane, ring expanding to form the cyclic peroxide which then decomposes; (2) attack onto the central single bond of the diyne which then fissions to form two C(5)H(5)O radicals; (3) attack on the alkyne-cyclopropane moiety to form a seven-membered ring peroxide which then decomposes. Each of these unimolecular combustion processes releases energy that induces additional radicals to accelerate the combustion process. Here oxygen has major effects both as the radical acceptor and as the radical producer. We extract both the effective activation energy and the effective pre-exponential factor by kinetic analysis of pyrolysis and combustion from these ReaxFF simulations. The low value of the derived effective activation energy (26.18 kcal/mol for pyrolysis and 16.40 kcal/mol for combustion) reveals the high activity of this fuel molecule.

The proton conductivity and the mechanical property of the polymer electrolyte membranes (PEMs) depend heavily on the equilibrium water content of the polymer systems. Molecular simulation method was used to investigate the equilibrium solubility and diffusion of water in hydrated Nafion 117. The simulations were based on the ab initio TEAM force field that was derived based on quantum mechanics data, liquid density and heat of vaporization of model compounds. The combination of the thermodynamic‐integration approach and Widom’s particle‐insertion method was carried out to determine the excess chemical potential of water in the PEMs and then to estimate of their water sorption behavior. The dynamics of water molecules and hydronium ions in PEMs were investigated using molecular dynamics simulations. The calculated diffusion coefficients were in good agreement with experimental. These results should be useful in guiding the development of other membranes to replace Nafion in PEMs fuel cells.

■ ASSOCIATED CONTENT *S Supporting Information H2O generation curves of all 19 samples, H2O generation curves of ΔV = 50, 60, 70 and 80, potential energy and force curves, H2 loss curve at 2498 K, H2O2 generation curve at 2498 K, species generated in aARRDyn simulations with ΔV = 20 and 40 kcal at 2498 K; the new ReaxFF−OH2014 force field parameters plus some of the validations against QM; results for the Arrhenius analysis, reaction mechanisms and kinetics from the older ReaxFF-CHO2008 force field, the force field file, and the plot script. This material is available free of charge via the Internet at http://pubs.acs.org.

Free energy perturbation methods were applied to predict water contents in hydrated poly(perfluorosulfonic acids) (PPFSA). The simulations were based on the TEAM force field which was derived from quantum mechanical data calculated for small molecules using density functional theory (DFT) and thermodynamic data of molecular liquids and crystal. The equilibrium water contents in three PPFSA polymers (Nafion-117, Nafion-115 and Hyflon) were predicted by evaluating excess chemical potentials of water in hydrated polymers and in pure water. High level of precision measured by average uncertainty of ca. 0.1 kcal mol(-1), and accuracy in terms of deviation from experimental data by ca. 0.2 kcal mol(-1) were obtained in the predicted excess chemical potentials. The predicted amounts of water uptake agree well with experimental values. In addition, the equilibrium and dynamic properties of hydrated Nafion-117 were calculated and the results agree well with the existing experimental and computational data. The entropy and enthalpy contributions in the calculated excess chemical potentials are analyzed and the results are consistent with intuition. A linear correlation between the entropies and enthalpies is identified for the systems studied, which indicates that just increasing the interaction energies between water and host materials does not guarantee enhancement of the water uptake.

Non-equilibrium molecular dynamics simulations are performed to calculate shear viscosities of 16 representative molecular liquids using the periodic perturbation method (PPM). A perturbation index is defined to measure the strength of the perturbation. It is identified that the predictions are systematically underestimated using PPM. The origin of the underestimate is the acoustic wave in the liquid density, which is persistent in the simulation box unless the perturbation is completely removed. However, there is a linear correlation between the perturbation indexes and the apparent viscosities, which can be utilized to accurately predict the shear viscosities. Finally, it is demonstrated that general force fields derived based on equilibrium properties can be used to predict the shear viscosities of small molecular liquids with relative errors less than 10%.