Robust three-body water simulation model

The most common potentials used in classical simulations of liquid water assume a pairwise additive form. Although these models have been very successful in reproducing many properties of liquid water at ambient conditions, none is able to describe accurately water throughout its complicated phase diagram. The primary reason for this is the neglect of many-body interactions. To this end, a simulation model with explicit three-body interactions was introduced recently [R. Kumar and J. L. Skinner, J. Phys. Chem. B 112, 8311 (2008)]. This model was parameterized to fit the experimental O-O radial distribution function and diffusion constant. Herein we reparameterize the model, fitting to a wider range of experimental properties (diffusion constant, rotational correlation time, density for the liquid, liquid/vapor surface tension, melting point, and the ice Ih density). The robustness of the model is then verified by comparing simulation to experiment for a number of other quantities (enthalpy of vaporization, dielectric constant, Debye relaxation time, temperature of maximum density, and the temperature-dependent second and third virial coefficients), with good agreement.     

Structural properties of a calcium aluminosilicate glass from molecular-dynamics simulations: a finite size effects study

We study a calcium aluminosilicate glass of composition (SiO(2))(0.67)-(Al(2)O(3))(0.12)-(CaO)(0.21) by means of molecular-dynamics simulations, using a potential made of two-body and three-body interactions. In order to prepare small samples that can subsequently be studied by first principles, the finite size effects on the liquid dynamics and on the glass structural properties are investigated. We find that finite size effects affect the Si-O-Si and Si-O-Al angular distributions, the first peaks of the Si-O, Al-O, and Ca-O pair correlation functions, the Ca coordination, and the oxygen atoms' environment in the smallest system (100 atoms). We give evidence that these finite size effects can be directly attributed to the use of three-body interactions.     

基于两分子和三分子相互作用的流体界面张力研究

提出了一个改进的密度泛函理论模型.该模型同时考虑了流体中两分子和三分子相互作用对体系Helm hlotz自由能的贡献,对Ar,N2,CH4,CO2等四种流体的气液界面张力进行了预测,结果与实验值均吻合良好.通过与仅考虑两分子作用的理论计算值和分子模拟值进行比较,表明流体中的三分子相互作用对描述非均相流体的结构与性质有重要影响.     

van der Waals forces between nanoclusters: importance of many-body effects

van der Waals interactions between nanoclusters have been calculated with a self-consistent, coupled dipole method. The method accounts for all many-body (MB) effects. Comparison is made between the exact potential energy, V, and the values obtained with two alternative methods: the sum of two-body interactions and the sum of two-body and three-body interactions. For all cases considered, the three-body term alone does not accurately represent the MB contributions to V. MB contributions are especially large for shape-anisotropic clusters.     

Quantum computation of the properties of helium using two-body and three-body intermolecular potentials: a molecular dynamics study

We have performed the molecular dynamics simulation to obtain energy, pressure, and self-diffusion coefficient of helium at different temperatures and densities using Lennard–Jones (LJ), Hartree–Fock dispersion-Individual damping (HFD-ID) potential, and the HFD-like potential which has been obtained with an inversion of viscosity data at zero pressure supplemented by quantum corrections following the Feynman–Hibbs approach. The contribution of three-body interactions using an accurate simple relationship reported by Wang and Sadus between two-body and three-body interactions has been also involved for non-effective potentials (HFD-ID and HFD-like) in simulation. Our results show a good agreement with corresponding experimental data. A comparison of our simulated results with other molecular simulations using different potentials is also included.     

Relationships between three-body and two-body interactions in fluids and solids

Molecular dynamics data are reported for two-body and three-body interactions in noble gases at densities covering the gas, liquid, and solid phases. The data indicate that simple relationships exist between three- and two-body interactions in both fluid and solid phases. The relationship for liquids has a simple density dependence with only one external parameter. In contrast, the solid phase relationship depends both on density and on the square of density and requires the evaluation of two parameters. The relationships are tested for both system-size and temperature dependences. The values of the relationship parameters are only sensitive to system size when a small number of atoms are involved. For 500 or more atoms, they remain nearly constant. The relationships are valid for both subcritical and slightly supercritical temperatures. A practical benefit of the relationships is that they enable the use of two-body intermolecular potentials for the prediction of the properties of real systems without the computational expense of three-body calculations.     

Two- and three-body interactions among nanoparticles in a polymer melt

We perform direct three-dimensional density functional theory (DFT) calculations of two- and three-body interactions in polymer nanocomposites. The nanoparticles are modeled as hard spheres, immersed in a hard-sphere homopolymer melt of freely jointed chains. The two-particle potential of mean force obtained from the DFT is in near quantitative agreement with the potential of mean force obtained from self-consistent polymer reference interaction site model theory. Three-body interactions among three nanoparticles are found to be significant, such that it is not possible to describe these systems with a polymer-mediated two-body interaction calculated from the potential of mean force.     

A practical treatment for the three-body interactions in the transcorrelated variational Monte Carlo method: application to atoms from lithium to neon

We suggest a practical solution to dealing with the three-body interactions in the transcorrelated variational Monte Carlo method (TC-VMC). In the TC-VMC method, which was suggested in our previous paper [N. Umezawa and S. Tsuneyuki, J. Chem. Phys. 119, 10015 (2003)], the Jastrow-Slater-type wave function is efficiently optimized through a self-consistent procedure by minimizing the variance of the local energy. The three-body terms in the transcorrelated self-consistent-field equation, which have been simply ignored in our previous works, are efficiently calculated by the Monte Carlo numerical integration. We found that our treatment for the three-body interactions is successful for atoms from Li to Ne.     

水团簇(H2O)n中多体作用强度与水分子间距的关系

使用高精度从头算方法(含基组重叠误差校正)计算了水团簇(H₂O)_n (n = 8, 10, 16, 20, 22, 24)中的所有二体、三体和四体作用能,分析了水团簇中的多体效应.研究表明,二体作用对体系总作用能的贡献高达70%以上,三体作用对总作用能的贡献可高达25%,四体作用在总作用能中所占比例不超过3%,五体及以上多体作用能在总作用能中所占比例更小,不超过0.5%.本文研究还表明,两个水分子间距小于0.68 nm的二体作用、三个和四个水分子中最近的两个水分子间距小于0.31 nm的三体和四体作用对体系总作用能的贡献高达99.4%.因此,以生物体系为对象的分子模拟方法应该具备准确地模拟两个水分子间距小于0.68 nm的二体作用、三个和四个水分子中最近的两个分子间距小于0.31 nm的三体和四体作用的能力.     

Quantum vibrational analysis and infrared spectra of microhydrated sodium ions using an ab initio potential

We present a full-dimensional potential energy surface and a dipole moment surface (DMS) for hydrated sodium ion. These surfaces are based on an n-body expansion for both the potential energy and the dipole moment, truncated at the two-body level for the H(2)O-Na(+) interaction and also for the DMS. The water-water interaction is truncated at the three-body level. The new full-dimensional two-body H(2)O-Na(+) potential is a fit to roughly 20,000 coupled-cluster single double (triple)/aug-cc-pVTZ energies. Properties of this two-body potential and the potential describing (H(2)O)(n)Na(+) clusters, with n up to 4 are given. We then report anharmonic, coupled vibrational calculations with the "local-monomer model" to obtain infrared spectra and also 0 K radial distribution functions for these clusters. Some comparisons are made with the recent infrared predissociation spectroscopy experiments of Miller and Lisy [J. Am. Chem. Soc. 130, 15381 (2008).].     

Molecular dynamics simulations on the local order of liquid and amorphous ZnTe

Molecular dynamics studies of structural and dynamical correlations of molten and vitreous states under several conditions of density and temperature were performed. We use an effective recently proposed interatomic potential, consisting of two- and three-body covalent interactions which has successfully described the structural, dynamical, and structural phase transformation induced by pressure in ZnTe [D. S. Borges and J. P. Rino, Phys. Rev. B 72, 014107 (2005)]. The two-body term of the interaction potential consists of Coulomb interaction resulting from charge transfer, steric repulsion due to atomic sizes, charge-dipole interaction to include the effect of electronic polarizability of anions, and dipole-dipole (van der Waals) interactions. The three-body covalent term is a modification of the Stillinger-Weber potential. Molecular dynamics simulations in isobaric-isenthalpic ensemble have been performed for systems amounting to 4096 and 64 000 particles. Starting from a crystalline zinc-blende (ZB) structure, the system is initially heated until a very homogeneous liquid is obtained. The vitreous zinc telluride phase is attained by cooling the liquid at sufficiently fast cooling rates, while slower cooling rates lead to a disordered ZB crystalline structure. Two- and three-body correlations for the liquid and vitreous phases are analyzed through pair distribution functions, static structure factors, and bond angle distributions. In particular, the neutron static structure factor for the liquid phase is in very good agreement with both the reported experimental data and first-principles simulations.     

Calculation of argon trimer rovibrational spectrum

Rovibrational spectra of Ar3 are computed for total angular momenta up to J=6 using row-orthonormal hyperspherical coordinates and an expansion of the wave function on hyperspherical harmonics. The sensitivity of the spectra to the two-body potential and to the three-body corrections is analyzed. First, the best available semiempirical pair potential (HFDID1) is compared with our recent ab initio two-body potential. The ab initio vibrational energies are typically 1-2 cm-1 higher than the semiempirical ones, which is related to the slightly larger dissociation energy of the semiempirical potential. Then, the Axilrod-Teller asymptotic expansion of the three-body correction is compared with our newly developed ab initio three-body potential. The difference is found smaller than 0.3 cm-1. In addition, we define approximate quantum numbers to describe the vibration and rotation of the system. The vibration is represented by a hyper-radial mode and a two-degree-of-freedom hyperangular mode, including a vibrational angular momentum defined in an Eckart frame. The rotation is described by the total angular momentum quantum number, its projection on the axis perpendicular to the molecular plane, and a hyperangular internal momentum quantum number, related to the vibrational angular momentum by a transformation between Eckart and principal-axes-of-inertia frames. These quantum numbers provide a qualitative understanding of the spectra and, in particular, of the impact of the nuclear permutational symmetry of the system (bosonic with zero nuclear spin). Rotational constants are extracted from the spectra and are shown to be accurate only for the ground hyperangular mode.     

Ionic solvation studied by image-charge reaction field method

In a preceding paper [J. Chem. Phys. 131, 154103 (2009)], we introduced a new, hybrid explicit/implicit method to treat electrostatic interactions in computer simulations, and tested its performance for liquid water. In this paper, we report further tests of this method, termed the image-charge solvation model (ICSM), in simulations of ions solvated in water. We find that our model can faithfully reproduce known solvation properties of sodium and chloride ions. The charging free energy of a single sodium ion is in excellent agreement with the estimates by other electrostatics methods, while offering much lower finite-size errors. Similarly, the potentials of mean force computed for Na-Cl, Na-Na, and Cl-Cl pairs closely reproduce those reported previously. Collectively, our results demonstrate the superior accuracy of the proposed ICSM method for simulations of mixed media.     

Comparison of MNDO to tight-binding,total-energy methods for surface atomic structure determination: Aluminum phosphide (110)

Tight-binding, total-energy (TBTE ) models have been successfully used to calculate the equilibrium surface atomic structures of a variety of materials, but are difficult to apply to substances with complicated interatomic repulsions. In these cases, the modified neglect of diatomic overlap (MNDO ) method, which specifically includes longer-range interactions, may provide an effective alternative. We present new calculations of the surface atomic structure of the AlP (110) surface using both techniques. This surface, whose structure has been determined quantitatively using a low-energy, electron-diffraction intensity analysis, displays a well-known relaxation characteristic of (110) surfaces of zincblende-structure semiconductors. The TBTE model, parameterized to bulk AlP properties, provides a more accurate prediction of the relaxed surface atomic positions, although MNDO , parameterized to small molecules, produces acceptable results. Despite the greater demands placed by MNDO on computer resources, it may prove useful in the study of materials that are difficult to model within a TBTE framework. © 1992 John Wiley & Sons, Inc.     

Asymptotic separability of three-body continuum wave functions for Coulomb systems

We consider a general three-body Coulomb system above the threshold for total break-up. For large particle separations the Schrödinger equation for this system is shown to be separable in terms of new suitably defined 6-dimensional parabolic coordinates. Although these coordinates are not orthogonal, mixed second derivative terms in the expression for the kinetic energy do not contribute at large particle separations to the Coulomb modification of free particle states. Rejection of mixed second derivatives at all particle configurations yields a three-body continuum wave function that satisfies exact asymptotic boundary conditions for Coulomb systems and treats all two-body interactions fully symmetric. This analysis splits the wave equation into an ‘unperturbed’ part plus a perturbation where the unperturbed part takes into account all long-range correlations.     

Viabilty of atomistic potentials for thermodynamic properties of carbon dioxide at low temperatures

Investigation into volumetric and energetic properties of several atomistic models mimicking carbon dioxide geometry and quadrupole momentum covered the liquid-vapor coexistence curve. Thermodynamic integration over a polynomial and an exponential-polynomial path was used to calculate free energy. Computational results showed that model using GROMOS Lennard-Jones parameters was unsuitable for bulk CO(2) simulations. On the other hand, model with potential fitted to reproduce only correct density-pressure relationship in the supercritical region proved to yield correct enthalpy of vaporization and free energy of liquid CO(2) in the low-temperature region. Except for molar volume at the upper part of the vapor-liquid equilibrium line, the bulk properties of exp-6-1 parametrization of ab initio CO(2) potential were in a close agreement with the experimental results. Copyright 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1772-1781, 2001     

Monte Carlo liquid water simulation with four-body interactions included

A four-body interaction potential for water molecules is derived. The new terms are combined with previously developed two- and three-body potential terms and applied in a Metropolis—Monte Carlo simulation at 298 K for an (N, V, T) ensemble with 512 water molecules. Improvements, relative to the results using only two-body, or two- and three-body interactions, are reported for the correlation function g(OO), for the X-ray and neutron-beam scattering intensities, and for the enthalpy.