Melting curve for argon calculated from pure theory

Revisiting the pseudo-supercritical path method: An improved formulation for the alchemical calculation of solid-liquid coexistence

Alchemical free energy calculations via molecular dynamics have been applied to obtain thermodynamic properties related to solid-liquid equilibrium conditions, such as melting points. In recent years, the pseudo-supercritical path (PSCP) method has proved to be an important approach to melting point prediction due to its flexibility and applicability. In the present work, we propose improvements to the PSCP alchemical cycle to make it more compact and efficient through a concerted evaluation of different potential energies. The multistate Bennett acceptance ratio (MBAR) estimator was applied at all stages of the new cycle to provide greater accuracy and uniformity, which is essential concerning uncertainty calculations. In particular, for the multistate expansion stage from solid to liquid, we employed the MBAR estimator with a reduced energy function that allows affine transformations of coordinates. Free energy and mean derivative profiles were calculated at different cycle stages for argon, triazole, propenal, and the ionic liquid 1-ethyl-3-methyl-imidazolium hexafluorophosphate. Comparisons showed a better performance of the proposed method than the original PSCP cycle for systems with higher complexity, especially the ionic liquid. A detailed study of the expansion stage revealed that remapping the centers of mass of the molecules or ions is preferable to remapping the coordinates of each atom, yielding better overlap between adjacent states and improving the accuracy of the methodology.

Thermodynamic properties of krypton from Monte Carlo simulations using ab initio potentials

Ten different thermodynamic properties of the noble gas krypton were calculated by Monte Carlo simulations in the isothermal-isobaric ensemble using a highly accurate ab initio pair potential, Feynman-Hibbs corrections for quantum effects, and an extended Axilrod-Teller-Muto potential to account for nonadditive three-body interactions. Fourteen state points at a liquid and a supercritical isotherm were simulated. To obtain results representative for macroscopic systems, simulations with several particle numbers were carried out and extrapolated to the thermodynamic limit. Our results agree well with experimental data from the literature, an accurate equation of state for krypton, and a recent virial equation of state (VEOS) for krypton in the region where the VEOS has converged. These results demonstrate that very good agreement between simulation and experiment can only be achieved if nonadditive three-body interactions and quantum effects are taken into account.

Thermodynamic properties of argon from Monte Carlo simulations using ab initio potentials

Ten different thermodynamic properties of the noble gas argon in the liquid and supercritical regions were obtained from semiclassical Monte Carlo simulations in the isothermal-isobaric ensemble using ab initio potentials for the two-body and nonadditive three-body interactions. Our results for the density and speed of sound agree with the most accurate experimental data for argon almost within the uncertainty of these data, a level of agreement unprecedented for many-particle simulations. This demonstrates the high predictive but yet unexploited power of ab initio potentials in the field of molecular modeling and simulation for thermodynamic properties of fluids.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

Rechargeable batteries have become indispensable implements in our daily life and are considered a promising technology to construct sustainable energy systems in the future. The liquid electrolyte is one of the most important parts of a battery and is extremely critical in stabilizing the electrode-electrolyte interfaces and constructing safe and long-life-span batteries. Tremendous efforts have been devoted to developing new electrolyte solvents, salts, additives, and recipes, where molecular dynamics (MD) simulations play an increasingly important role in exploring electrolyte structures, physicochemical properties such as ionic conductivity, and interfacial reaction mechanisms. This review affords an overview of applying MD simulations in the study of liquid electrolytes for rechargeable batteries. First, the fundamentals and recent theoretical progress in three-class MD simulations are summarized, including classical, ab initio, and machine-learning MD simulations (section 2). Next, the application of MD simulations to the exploration of liquid electrolytes, including probing bulk and interfacial structures (section 3), deriving macroscopic properties such as ionic conductivity and dielectric constant of electrolytes (section 4), and revealing the electrode-electrolyte interfacial reaction mechanisms (section 5), are sequentially presented. Finally, a general conclusion and an insightful perspective on current challenges and future directions in applying MD simulations to liquid electrolytes are provided. Machine-learning technologies are highlighted to figure out these challenging issues facing MD simulations and electrolyte research and promote the rational design of advanced electrolytes for next-generation rechargeable batteries.

A Molecular Dynamics Study of Monomer Melt Properties of Cyanate Ester Monomer Melt Properties

The objective of this work was to computationally predict the melting temperature and melt properties of thermosetting monomers used in aerospace applications. In this study, we applied an existing voids method by Solca. to examine four cyanate ester monomers with a wide range of melting temperatures. Voids were introduced into some simulations by removal of molecules from lattice positions to lower the free-energy barrier to melting to directly simulate the transition from a stable crystal to amorphous solid and capture the melting temperature. We validated model predictions by comparing melting temperature against previously reported literature values. Additionally, the torsion and orientational order parameters were used to examine the monomers’ freedom of motion to investigate structure–property relationships. Ultimately, the voids method provided reasonable estimates of melting temperature while the torsion and order parameter analysis provided insight into sources of the differing melt properties between the thermosetting monomers. As a whole, the results shed light on how freedom of molecular motions in the monomer melt state may affect melting temperature and can be utilized to inspire the development of thermosetting monomers with optimal monomer melt properties for demanding applications.

Rigorous expressions for thermodynamic properties in the NpH ensemble

Molecular expressions for thermodynamic properties of fluids and derivatives of the entropy up to third order in the isoenthalpic-isobaric ensemble are derived by using the methodology developed by Lustig for the microcanonical and canonical ensembles [J. Chem. Phys. 100, 3048 (1994)JCPSA60021-960610.1063/1.466446; Mol. Phys. 110, 3041 (2012)MOPHAM0026-897610.1080/00268976.2012.695032]. They are expressed in a systematic way by phase-space functions, which represent derivatives of the phase-space volume with respect to enthalpy and pressure. The expressions for thermodynamic properties contain only ensemble averages of combinations of the kinetic energy and volume of the system. Thus, the calculation of thermodynamic properties in the isoenthalpic-isobaric ensemble does not require volume derivatives of the potential energy. This is particularly advantageous in Monte Carlo simulations when the interactions between molecules are described by very accurate ab initio pair and nonadditive three-body potentials. The derived expressions are validated by Monte Carlo simulations for the simple Lennard-Jones model fluid as a test case.

Ab initioinvestigation of the temperature-dependent elastic properties of Bi, Te and Cu

Using density functional theory andab initiomolecular dynamics, we have investigated the elastic properties of Bi, Te and Cu as a function of temperature. We compare calculated quantities which can be used to determine the effectiveness of our proposed method, such as the bulk (K), shear (G), and Young's (E) moduli. We also computed Poisson's ratio (ν) and the Pugh ratio (γ) for each of these materials at different temperatures to investigate changes in ductility. We have used the elastic moduli to calculate the Debye temperatureθDand minimum thermal conductivitykminof these materials as a function of temperature. We found that the elastic properties calculated in this work are in good agreement with experimental work. The inclusion of temperature effects has allowed for the proper prediction of ductility for each of these materials, a feat that standard density functional theory calculations has previously been unable to accomplish for Bi and Te.

Addressing the discrepancy of finding the equilibrium melting point of silicon using molecular dynamics simulations

We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid-liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon.

A comparison of methods for melting point calculation using molecular dynamics simulations

Accurate and efficient prediction of melting points for complex molecules is still a challenging task for molecular simulation, although many methods have been developed. Four melting point computational methods, including one free energy-based method (the pseudo-supercritical path (PSCP) method) and three direct methods (two interface-based methods and the voids method) were applied to argon and a widely studied ionic liquid 1-n-butyl-3-methylimidazolium chloride ([BMIM][Cl]). The performance of each method was compared systematically. All the methods under study reproduce the argon experimental melting point with reasonable accuracy. For [BMIM][Cl], the melting point was computed to be 320 K using a revised PSCP procedure, which agrees with the experimental value 337-339 K very well. However, large errors were observed in the computed results using the direct methods, suggesting that these methods are inappropriate for large molecules with sluggish dynamics. The strengths and weaknesses of each method are discussed.

Semi-Empirical Methods to Predict the Physical Properties of Ionic Liquids: An Overview of Recent Developments

The field of ionic liquids (ILs) has shown rapid growth in recent years. Much of this work has involved the synthesis of new ILs and their application in an ever-increasing number of areas. In contrast, there have been relatively few studies that investigate and attempt to predict the fundamental physical properties of ILs, which are extremely important for their applications. The quantitative prediction of the physical properties of unknown salts remains an important goal in IL research. This will allow the design of new ILs with specific properties tailored for particular applications, without the need for time-consuming trial and error syntheses. Recently, several studies have shown that it is possible to make predictions of the physical properties of ILs e.g. melting points, conductivities, viscosities, densities, surface tensions and refractive indices. This paper gives an overview of these semi-empirical methods and makes some comparisons regarding the accuracy of their predictions and their applicability to predicting the properties of unknown salts.

Monte Carlo simulation in the isobaric-multithermal ensemble of a bulk Lennard-Jones fluid system: thermodynamic quantities for pressure from P{ *}=2.42 to 7.25

A Monte Carlo (MC) simulation in the isobaric-multithermal (MuTh) ensemble has been carried out for a bulk Lennard-Jones fluid system that consists of 108 particles. The MuTh weight factor which we determined turned out to be reliable for the temperature and pressure range from (T{ *},P{ *})=(T_{0}{ *},P_{0}{ *})=(2.09,7.25) to (T{ *},P{ *})=(0.417,1.45) along P{ *}/T{ *}=P_{0}{ *}/T_{0}{ *} . Thermodynamic quantities calculated from the MuTh MC production run by the reweighting techniques showed the discontinuous change, which is the feature of the first-order phase transition. The radial distribution functions suggest that the transition takes place between liquid and solid states. Two distinct local maxima were observed in the obtained contour representations of the probability distribution at T{ *}=1.04 and P{ *}=3.63 , which is the phase transition point along P{ *}/T{ *}=P_{0}{ *}/T_{0}{ *} .

Molecular dynamics simulations of the melting mechanisms of perfect and imperfect crystals of dimethylnitramine

The melting mechanisms of perfect and imperfect crystalline dimethylnitramine have been studied using molecular dynamics simulations. The imperfect crystal was created by removing approximately 10% of the molecules from the center of the simulation cell. The density, diffusion coefficient, translational and orientational order parameters, and void size were calculated as functions of temperature and simulation time. Upon melting, the volume of the imperfect crystal slowly decreases with time due to the shrinkage of the void then suddenly decreases to a minimum value due to collapse of the structure around the void with concomitant diffusion of molecules into the void. The simulation cell volume then increases as the liquid nucleus formed at the void expands. The melting of perfect crystals must occur by a different mechanism. As the temperature of the perfect crystal reaches the maximum superheating temperature, there is an increase in the thermal motions of the molecules that result in the formation of liquid centers (characterized by translational order parameter consistent with the liquid phase) at random locations. The liquid centers rapidly grow, resulting in a complete transition to the liquid phase. The increases in orientational and translational freedom occur simultaneously in the imperfect crystal, and in the perfect crystal, orientational freedom significantly precedes translational freedom.

On the Accuracy of Force Fields for Predicting the Physical Properties of Dimethylnitramine

The accuracy of three force fields for predicting the physical properties of dimethylnitramine (DMNA) has been investigated by using molecular dynamics simulations. The Sorescu, Rice, and Thompson (SRT) (J. Phys. Chem. B 1997, 101, 798) rigid-molecule, flexible generalized AMBER (J. Comput. Chem. 2004, 25, 1157), and Smith et al. flexible force fields (J. Phys. Chem. B 1999, 103, 705) were tested. The density, lattice parameters, isotherm, and melting point of DMNA are calculated using classical molecular dynamics. Except for the melting point, the predictions of the three force fields are in reasonable agreement with experimental values. The calculated thermodynamic melting points (Tmp) for the SRT, AMBER, and Smith et al. force fields are 380, 360, and 260 K, respectively. The experimental value is 331 K. Modifications of the torsional barriers in the AMBER force field resulted in Tmp = 346 K, in good agreement with the experimental value of 331 K. The calculated lattice parameters and bulk modulus are also improved with the modifications of the AMBER potential. The results indicate that, although not sufficiently accurate without modifications, the general force fields such as AMBER provide the basis for developing force fields that correctly predict the physical properties of nitramines.

Molecular dynamics simulations of melting and the glass transition of nitromethane

Molecular dynamics simulations have been used to investigate the thermodynamic melting point of the crystalline nitromethane, the melting mechanism of superheated crystalline nitromethane, and the physical properties of crystalline and glassy nitromethane. The maximum superheating and glass transition temperatures of nitromethane are calculated to be 316 and 160 K, respectively, for heating and cooling rates of 8.9 x 10(9) Ks. Using the hysteresis method [Luo et al., J. Chem. Phys. 120, 11640 (2004)] and by taking the glass transition temperature as the supercooling temperature, we calculate a value of 251.1 K for the thermodynamic melting point, which is in excellent agreement with the two-phase result [Agrawal et al., J. Chem. Phys. 119, 9617 (2003)] of 255.5 K and measured value of 244.73 K. In the melting process, the nitromethane molecules begin to rotate about their lattice positions in the crystal, followed by translational freedom of the molecules. A nucleation mechanism for the melting is illustrated by the distribution of the local translational order parameter. The critical values of the Lindemann index for the C and N atoms immediately prior to melting (the Lindemann criterion) are found to be around 0.155 at 1 atm. The intramolecular motions and molecular structure of nitromethane undergo no abrupt changes upon melting, indicating that the intramolecular degrees of freedom have little effect on the melting. The thermal expansion coefficient and bulk modulus are predicted to be about two or three times larger in crystalline nitromethane than in glassy nitromethane. The vibrational density of states is almost identical in both phases.

Molecular Dynamics Simulations of the Melting of 1,3,3-Trinitroazetidine

Physical properties of condensed-phase 1,3,3-trinitroazetidine (TNAZ) have been computed with molecular dynamics (MD) and a nonreactive, fully flexible force field formulated by combining the intramolecular interactions obtained from the Generalized AMBER Force Field and the rigid-molecule force field developed by Sorescu-Rice-Thompson [J. Phys. Chem. B 1997, 101, 798] (AMBER-SRT). The results are compared with MD calculations, using the AMBER force field. The predicted densities of crystalline TNAZ from both force fields are about 10% lower than the experimental value. The calculated thermodynamic melting point at 1 atm from the AMBER-SRT force field is 390 K, in good agreement with the measured value of 374 K, while the AMBER force field predicts a thermodynamic melting point of 462 K. The lattice parameters and the molecular and crystal structures calculated with the AMBER-SRT force field are in excellent agreement with experiment. Simulations with the AMBER-SRT force field were also used to generate the isotherm of TNAZ up to 4 GPa and the bulk modulus and its pressure derivative.

Simulations of the Solid, Liquid, and Melting of 1-n-Butyl-4-amino-1,2,4-triazolium Bromide

Molecular dynamics simulations are used to study the solid and liquid properties and to predict the melting point of 1-n-propyl-4-amino-1,2,4-triazolium bromide ([patr][Br]) using a force field based on the one developed by Canongia Lopes et al. (J. Phys. Chem. B 2004, 108, 2038) for dialkyl substituted imidazolium salts, which was modified by including terms from the general AMBER force field. Electrostatic charges for the intermolecular interactions were determined from gas-phase ab initio electron structure calculations of the triazolium cation. Simulations of the solid state at 100 K reproduced the experimental density to within 4%. Simulations from 100 K to the melting point and the liquid from 333 to 500 K were performed to determine the temperature dependence of the densities of the two phases. The structures of the solid and liquid phases are characterized with radial distribution functions, which show that there are strong spatial correlations among neighboring ion pairs in liquid [patr][Br]. The dynamic behavior of the ions in the liquid state is also studied by computing velocity autocorrelation functions and the mean-square displacements between the ions. The melting point is determined by simulating void-induced melting. Changes in the density, intermolecular energy, and Lindemann index are used as indicators of the melting transition. The computed melting point is 360 +/- 10 K, which is within 10% of the experimental value 333 K.

Molecular dynamics studies of melting and some liquid-state properties of 1-ethyl-3-methylimidazolium hexafluorophosphate [emim][PF6]

Molecular dynamics simulations are used to study the liquid-state properties and melting of 1-ethyl-3-methylimidazolium hexafluorosphosphate [emim][PF6] using the force field of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] and geometric constants from crystallographic data. The structures of the solid and liquid states are characterized by carbon-carbon, carbon-phosphorous, and phosphorous-phosphorous radial distribution functions. Spatial correlations among the ions are strong in the liquid state. The cohesive energy density and the temperature dependences of the molar volume and density of the liquid have been computed. The melting point is determined by equilibrating the solid-state supercells in which void defects have been introduced to eliminate the free-energy barrier for the formation of a solid-liquid interface. The computed melting point is 375+/-10 K, which is approximately 10% higher than the experimental value of 333 K.

Toward a robust and general molecular simulation method for computing solid-liquid coexistence

A rigorous and generally applicable method for computing solid-liquid coexistence is presented. The method overcomes some of the technical difficulties associated with other solid-liquid simulation procedures and can be implemented within either a molecular dynamics or Monte Carlo framework. The method consists of three steps: First, relative Gibbs free energy curves are created for the solid and liquid phases using histogram reweighting. Next, the free energy difference between the solid and liquid phases is evaluated at a single state point by integrating along a pseudosupercritical transformation path that connects the two phases. Using this result, the solid and liquid free energy curves are referenced to a common point, allowing a single coexistence point to be determined. Finally, Gibbs-Duhem integration is used to determine the full coexistence curve. To evaluate its utility, this method is applied to the Lennard-Jones and NaCl systems. Results for solid-liquid coexistence agree with previous calculations for these systems. In addition, it is shown that the NaCl model does not correctly describe solid-liquid coexistence at high pressures. An analysis of the accuracy of the method indicates that the results are most sensitive to the transformation free energy calculation.