Structure, thermodynamics, and liquid-vapor equilibrium of ethanol from molecular-dynamics simulations using nonadditive interactions

We present a molecular-dynamics simulation study of the bulk and liquid-vapor interfacial properties of ethanol using a polarizable force field based on the fluctuating charge (FQ) formalism, as well as the nonpolarizable CHARMM22 force field. Both models are competitive with respect to the prediction of ambient liquid properties such as liquid density, enthalpy of vaporization, dielectric constant, and self-diffusion constants. The polarizable model predicts an average condensed-phase dipole moment of 2.2 D associated with an induced liquid-phase dipole moment of 0.6 D; though qualitatively in agreement with earlier nonadditive models as well as recent Car-Parinello calculations, the current FQ model underestimates the condensed-phase dipole moment. In terms of liquid structure, both models are in agreement with recent neutron-diffraction results of liquid ethanol structure, although the polarizable model predicts the hydroxyl-hydrogen-hydroxyl-hydrogen structure factor in closer agreement with the experimental data. In terms of interfacial properties, both models predict ambient surface tension to within 4% of the experimental value of 22.8 dyncm, while overestimating the surface excess entropy by almost a factor of 2. Both models display the characteristic preferential orientation of interfacial molecules. The polarizable model allows for a monotonic variation of the average molecular dipole moment from the bulk value to that of the vapor phase. Consequently, there is a dramatic difference in the surface potential predicted by the polarizable and nonpolarizable models. The polarizable model estimates a surface potential of -209+/-3 mV, while the nonpolarizable model yields a value of -944+/-10 mV. Finally, based on the vapor-liquid equilibrium simulation data from several temperatures, we estimate the critical properties of both models. As observed with other FQ models for associating fluids (such as water and methanol), and counter to what one would anticipate by modeling more physically the electrostatic response to local environment, the current FQ model underestimates the critical temperature and overestimates the critical density of ethanol; moreover, the FQ model is, in this respect, equivalent to the underlying fixed-charge model. These results further suggest the need to revisit polarizable models in terms of quantitative vapor-liquid equilibrium prediction.     

Surface properties of the polarizable Baranyai-Kiss water model

The water surface properties using the Baranyai-Kiss (BK) model [A. Baranyai and P. T. Kiss, J. Chem. Phys. 133, 144109 (2010)] are studied by molecular dynamics simulation, and compared to popular rigid water potentials, namely to the extended simple point charge (SPC/E) and the transferable interaction potential with 4 points (TIP4P) models. The BK potential is a polarizable model of water with three Gaussian charges. The negative charge is connected to its field-free position by a classical harmonic spring, and mechanical equilibrium is established between this spring force and the force due to the charge distribution of the system. The aim of this study is, on the one hand, to test the surface properties of the new model, and on the other hand, to identify differences between the models listed above. The obtained results reveal that the BK model reproduces very well a number of properties corresponding to liquid-vapor equilibrium, such as the coexisting liquid and vapor densities, saturated vapor pressure or surface tension. Further, this model reproduces excellently the critical point of water even in comparison with a large number of widely used polarizable and nonpolarizable models. The structural properties of the liquid surface of BK water turns out to be very similar to that of the SPC/E model, while the surface of TIP4P water is found to be somewhat less ordered. This finding is related to the fact that the critical temperature of the TIP4P model is lower than that of either SPC/E or BK.     

Simulated surface tensions of common water models

Initial simulated values of the surface tension for the SPC/E water model have indicated excellent agreement with experiment. More recently, differing values have been obtained which are significantly lower than previous estimates. Here, we attempt to explain the differences between the previous studies and show that a variety of simulation conditions can affect the final surface tension values. Consistent values for the surface tensions of six common fixed charge water models (TIP3P, SPC, SPC/E, TIP4P, TIP5P, and TIP6P) are then determined for four temperatures between 275 and 350 K. The SPC/E and TIP6P models provide the best agreement with experiment.     

水分子团簇分子力场方法的比较研究

采用传统水分子力场模型(SPC, TIPnP(n=3-5))和极化模型(POL3, AMOEBA, SPC-FQ, TIP4P-FQ)对水分子二聚体团簇性质进行了比较和研究. 以从头计算和实验数据为依据, 分析水分子在外场作用下体系的静电极化, 电荷转移和分子结构变化. 通过水分子二聚体结合能和各分解能量项评价极化静电势能在双分子结合能中的地位和作用, 以及各水分子力场的适用性. 通过水分子团簇多体相互作用能的计算,展示不同极化水分子力场定量计算极化能量的实际能力. 通过对力场模型结果的对比和分析, 为进一步发展极化力场模型, 并应用到其他体系提供借鉴和依据.     

Effect of flexibility on surface tension and coexisting densities of water

Molecular dynamics simulations of pure water at the liquid-vapor interface are performed using direct simulation of interfaces in a liquid slab geometry. The effect of intramolecular flexibility on coexisting densities and surface tension is analyzed. The dipole moment profile across the liquid-vapor interface shows different values for the liquid and vapor phases. The flexible model is a polarizable model. This effect is minor for liquid densities and is large for surface tension. The liquid densities increase from 2% at 300 K to 9% at 550 K when the force field is changed from a fully rigid simple point charge extended (SPCE) model to that of a fully flexible model with the same intermolecular interaction parameters. The increases in surface tension at both temperatures are around 11% and 36%, respectively. The calculated properties of the flexible models are closer to the experimental data than those of the rigid SPCE. The effect of the maximum number of reciprocal vectors (h(z) (max)) and the surface area on the calculated properties at 300 K is also analyzed. The coexiting densities are not sensitive to those variables. The surface tension fluctuates with h(z) (max) with an amplitude larger than 10 mN m(-1). The effect of using small interfacial areas is slightly larger than the error in the simulations.     

甲烷水合物导热机理的分子动力学模拟

甲烷水合物导热系数是甲烷水合物勘探、开采、储运以及其他应用过程中一个十分重要的物理参数.我们采用平衡分子动力学（EMD）方法Green-Kubo理论计算温度203.15～263.15K、压力范围3～100MPa、晶穴占有率为0～1的sI甲烷水合物的导热系数,采用的水分子模型包括TIP4P、TIP4P-Ew、TIP4P-FQ、TIP4P/2005、TIP4P/Ice.研究了主客体分子、外界温压条件等对甲烷水合物导热性能的影响.研究结果显示甲烷水合物的低导热性能由主体分子构建的sI笼型结构决定,而客体分子进入笼型结构后,使得笼型结构导热性能增强,同时进入笼型结构的客体分子越多,甲烷水合物导热性能越强.研究结果还显示在高温区域（T〉TDebye/3）内不同温度作用下,所有sI水合物具有相似的导热规律.压力对导热系数有一定影响,尤其是在较高压力条件下,压力越高,导热系数越大.而在不同温度和不同压力作用过程中,密度的改变对导热系数的增大或减小几乎没有影响.     

CHARMM fluctuating charge force field for proteins: I parameterization and application to bulk organic liquid simulations

A first-generation fluctuating charge (FQ) force field to be ultimately applied for protein simulations is presented. The electrostatic model parameters, the atomic hardnesses, and electronegativities, are parameterized by fitting to DFT-based charge responses of small molecules perturbed by a dipolar probe mimicking a water dipole. The nonbonded parameters for atoms based on the CHARMM atom-typing scheme are determined via simultaneously optimizing vacuum water-solute geometries and energies (for a set of small organic molecules) and condensed phase properties (densities and vaporization enthalpies) for pure bulk liquids. Vacuum solute-water geometries, specifically hydrogen bond distances, are fit to 0.19 A r.m.s. error, while dimerization energies are fit to 0.98 kcal/mol r.m.s. error. Properties of the liquids studied include bulk liquid structure and polarization. The FQ model does indeed show a condensed phase effect in the shifting of molecular dipole moments to higher values relative to the gas phase. The FQ liquids also appear to be more strongly associated, in the case of hydrogen bonding liquids, due to the enhanced dipolar interactions as evidenced by shifts toward lower energies in pair energy distributions. We present results from a short simulation of NMA in bulk TIP4P-FQ water as a step towards simulating solvated peptide/protein systems. As expected, there is a nontrivial dipole moment enhancement of the NMA (although the quantitative accuracy is difficult to assess). Furthermore, the distribution of dipole moments of water molecules in the vicinity of the solutes is shifted towards larger values by 0.1-0.2 Debye in keeping with previously reported work.     

Structural properties and thermodynamics of water clusters: a Wang-Landau study

The temperature dependence of structural properties and thermodynamic behavior of water clusters has been studied using Wang-Landau sampling. Four potential models, simple point charge/extended (SPC/E), transferable intermolecular potential 3 point (TIP3P), transferable intermolecular potential 4 point (TIP4P), and Gaussian charge polarizable (GCP), are compared for ground states and properties at finite temperatures. Although the hydrogen bond energy and the distance of the nearest-neighbor oxygen pair are significantly different for TIP4P and GCP models, they approach to similar ground state structures and melting transition temperatures in cluster sizes we considered. Comparing with TIP3P, SPC/E model provides properties closer to that of TIP4P and GCP.     

Common features of orientational order at the temperature of maximum density for various water models: molecular dynamics study

Canonical ensembles for liquid water were obtained from molecular dynamics simulations at various temperatures using the TIP5P, TIP4P-FQ, TIP4P, and SPC/E water models at a fixed density of 1 g/cm3. From these ensembles, it was found that the distributions of the orientational order parameter q of these models showed similar patterns as temperature changed except that the distributions were shifted relative to each other by the difference of their temperature of maximum density (TMD). The four models exhibited similar distributions and average values of orientational order around their respective TMDs, and these common features were investigated in detail especially. The current study suggests that the unique microscopic configuration of water molecules cause TMD phenomenon in any reasonable water model. This finding provides a useful tool in the development of new water potentials by offering guidelines to predict the TMD, avoiding troublesome isothermal-isobaric ensemble simulations.     

Relation between the melting temperature and the temperature of maximum density for the most common models of water

Water exhibits a maximum in density at normal pressure at 4 degrees above its melting point. The reproduction of this maximum is a stringent test for potential models used commonly in simulations of water. The relation between the melting temperature and the temperature of maximum density for these potential models is unknown mainly due to our ignorance about the melting temperature of these models. Recently we have determined the melting temperature of ice I(h) for several commonly used models of water (SPC, SPC/E, TIP3P, TIP4P, TIP4P/Ew, and TIP5P). In this work we locate the temperature of maximum density for these models. In this way the relative location of the temperature of maximum density with respect to the melting temperature is established. For SPC, SPC/E, TIP3P, TIP4P, and TIP4P/Ew the maximum in density occurs at about 21-37 K above the melting temperature. In all these models the negative charge is located either on the oxygen itself or on a point along the H-O-H bisector. For the TIP5P and TIP5P-E models the maximum in density occurs at about 11 K above the melting temperature. The location of the negative charge appears as a geometrical crucial factor to the relative position of the temperature of maximum density with respect to the melting temperature.     

A classical polarizable model for simulations of water and ice

We develop a classical rigid polarizable model of water for molecular simulations of water and ice. The model uses the Rowlinson five-site geometry: oxygen bearing the Lennard-Jones interaction and linearly polarizable point dipole, two positively charged hydrogens, and two massless negative charges placed symmetrically off oxygen so that the experimental dipole moment is reproduced. The target properties are the densities of water and ice Ih, diffusivity, enthalpies of fusion and vaporization, and the ice Ih melting point. The surface tension at lower temperatures is by 7% underestimated whereas the dielectric constant by 6% overestimated. Diffusivity and viscosity worsen at higher temperatures, although the Stokes radius is overestimated only by 2-7%. The ice Ih melting temperature is 260 K and the temperature of maximum density is 269 K. Rescaling the charges by a factor of 1.01 and Lennard-Jones energy by 1.0201 improves the melting point and energy-related quantities but shifts the agreement of kinetic properties to higher temperatures. For the model we propose abbreviation POL4D.     

Efficient prediction of thermodynamic properties of quadrupolar fluids from simulation of a coarse-grained model: the case of carbon dioxide

Monte Carlo simulations are presented for a coarse-grained model of real quadrupolar fluids. Molecules are represented by particles interacting with Lennard-Jones forces plus the thermally averaged quadrupole-quadrupole interaction. The properties discussed include the vapor-liquid coexistence curve, the vapor pressure along coexistence, and the surface tension. The full isotherms are also accessible over a wide range of temperatures and densities. It is shown that the critical parameters (critical temperature, density, and pressure) depend almost linearly on a quadrupolar parameter q=Q(*4)T*, where Q* is the reduced quadrupole moment of the molecule and T* the reduced temperature. The model can be applied to a variety of small quadrupolar molecules. We focus on carbon dioxide as a test case, but consider nitrogen and benzene, too. Experimental critical temperature, density, and quadrupolar moment are sufficient to fix the parameters of the model. The resulting agreement with experiments is excellent and marks a significant improvement over approaches which neglect quadrupolar effects. The same coarse-grained model was also applied in the framework of perturbation theory in the mean spherical approximation. As expected, the latter deviates from the Monte Carlo results in the critical region, but is reasonably accurate at lower temperatures.     

Hydration free energies of monovalent ions in transferable intermolecular potential four point fluctuating charge water: an assessment of simulation methodology and force field performance and transferability

Hydration free energies of nonpolarizable monovalent atomic ions in transferable intermolecular potential four point fluctuating charge (TIP4P-FQ) are computed using several commonly employed ion-water force fields including two complete model sets recently developed for use with the simple water model with four sites and Drude polarizability and TIP4P water models. A simulation methodology is presented which incorporates a number of finite-system free energy corrections within the context of constant pressure molecular dynamics simulations employing the Ewald method and periodic boundary conditions. The agreement of the computed free energies and solvation structures with previously reported results for these models in finite droplet systems indicates good transferability of ion force fields from these water models to TIP4Q-FQ even when ion polarizability is neglected. To assess the performance of the ion models in TIP4P-FQ, we compare with consensus values for single-ion hydration free energies arising from recently improved cluster-pair estimates and a reevaluation of commonly cited, experimentally derived single-ion hydration free energies; we couple the observed consistency of these energies with a justification of the cluster-pair approximation in assigning single-ion hydration free energies to advocate the use of these consensus energies as a benchmark set in the parametrization of future ion force fields.     

The melting temperature of the most common models of water

The melting temperature of ice I(h) for several commonly used models of water (SPC, SPC/E,TIP3P,TIP4P, TIP4P/Ew, and TIP5P) is obtained from computer simulations at p = 1 bar. Since the melting temperature of ice I(h) for the TIP4P model is now known [E. Sanz, C. Vega, J. L. F. Abascal, and L. G. MacDowell, Phys. Rev. Lett. 92, 255701 (2004)], it is possible to use the Gibbs-Duhem methodology [D. Kofke, J. Chem. Phys. 98, 4149 (1993)] to evaluate the melting temperature of ice I(h) for other potential models of water. We have found that the melting temperatures of ice I(h) for SPC, SPC/E, TIP3P, TIP4P, TIP4P/Ew, and TIP5P models are T = 190 K, 215 K, 146 K, 232 K, 245 K, and 274 K, respectively. The relative stability of ice I(h) with respect to ice II for these models has also been considered. It turns out that for SPC, SPC/E, TIP3P, and TIP5P the stable phase at the normal melting point is ice II (so that ice I(h) is not a thermodynamically stable phase for these models). For TIP4P and TIP4P/Ew, ice I(h) is the stable solid phase at the standard melting point. The location of the negative charge along the H-O-H bisector appears as a critical factor in the determination of the relative stability between the I(h) and II ice forms. The methodology proposed in this paper can be used to investigate the effect upon a coexistence line due to a change in the potential parameters.     

Influence of the displacement out of the center of mass and nonaxiality of the dipole on the thermodynamics of liquids composed of linear dipole molecules

We present results for organic liquids modeled as linear rods with an embedded point dipole shifted from the geometrical center. Previously, we have obtained results for the vapor-liquid equilibrium (VLE) of similar systems with centered point dipoles. Our results included both models and applications to real systems. Results presented here are based on a previous work ( Phys. Rev. E 2003, 68, 021201) on the structural properties of these systems where relevant results about the appearance of dimers were found. Now, we have also performed systematic simulations on these systems to calculate the VLE of models with different aspect ratios, dipole shifts, and dipole strengths using the Gibbs ensemble Monte Carlo (GEMC) to calculate equilibrium densities and vapor pressure at each temperature. The applications considered here include some important substances such as 1-amines, acetonitrile, and 1-alcohols whose intermolecular parameters were fitted from our model simulations. Furthermore, we have used quantum chemistry calculations to obtain a reliable charge distribution, and we have applied our model to predict the vapor pressure of alpha,omega-diols where experimental results are rather scarce. Our results show a general improvement of the agreement between experiment and models compared to centered dipole models previously used. Results for amines are particularly remarkable.     

Influence of bond flexibility on the vapor-liquid phase equilibria of water

The authors performed Gibbs ensemble simulations on the vapor-liquid equilibrium of water to investigate the influence of incorporating intramolecular degrees of freedom in the simple point charge (SPC) water model. Results for vapor pressures, saturation densities, heats of vaporization, and the critical point for two different flexible models are compared with data for the corresponding rigid SPC and SPC/E models. They found that the introduction of internal vibrations, and also their parametrization, has an observable effect on the prediction of the vapor-liquid coexistence curve. The flexible SPC/Fw model, although optimized to describe bulk diffusion and dielectric constants at ambient conditions, gives the best prediction of saturation densities and the critical point of the examined models.     

Melting points and thermal expansivities of proton-disordered hexagonal ice with several model potentials

A method of free energy calculation is proposed, which enables to cover a wide range of pressure and temperature. The free energies of proton-disordered hexagonal ice (ice Ih) and liquid water are calculated for the TIP4P [J. Chem. Phys. 79, 926 (1983)] model and the TIP5P [J. Chem. Phys. 112, 8910 (2000)] model. From the calculated free energy curves, we determine the melting point of the proton-disordered hexagonal ice at 0.1 MPa (atmospheric pressure), 50 MPa, 100 MPa, and 200 MPa. The melting temperatures at atmospheric pressure for the TIP4P ice and the TIP5P ice are found to be about T(m)=229 K and T(m)=268 K, respectively. The melting temperatures decrease as the pressure is increased, a feature consistent with the pressure dependence of the melting point for realistic proton-disordered hexagonal ice. We also calculate the thermal expansivity of the model ices. Negative thermal expansivity is observed at the low temperature region for the TIP4P ice, but not for the TIP5P ice at the ambient pressure.     

Capillary waves at the liquid-vapor interface and the surface tension of water

Capillary waves occurring at the liquid-vapor interface of water are studied using molecular dynamics simulations. In addition, the surface tension, determined thermodynamically from the difference in the normal and tangential pressure at the liquid-vapor interface, is compared for a number of standard three- and four-point water models. We study four three-point models (SPC/E, TIP3P, TIP3P-CHARMM, and TIP3P-Ew) and two four-point models (TIP4P and TIP4P-Ew). All of the models examined underestimate the surface tension; the TIP4P-Ew model comes closest to reproducing the experimental data. The surface tension can also be determined from the amplitude of capillary waves at the liquid-vapor interface by varying the surface area of the interface. The surface tensions determined from the amplitude of the logarithmic divergence of the capillary interfacial width and from the traditional thermodynamic method agree only if the density profile is fitted to an error function instead of a hyperbolic tangent function.