Dynamic simulations of water at constant chemical potential

The grand molecular dynamics (GMD) method has been extended and applied to examine the density dependence of the chemical potential of a three-site water model. The method couples a classical system to a chemical potential reservoir of particles via an ansatz Lagrangian. Equilibrium properties such as structure and thermodynamics, as well as dynamic properties such as time correlations and diffusion constants, in open systems at a constant chemical potential, are preserved with this method. The average number of molecules converges in a reasonable amount of computational effort and provides a way to estimate the chemical potential of a given model force field

Reply to “Comment on ‘Phase diagram of MgO from density-functional theory and molecular-dynamics simulations’”

In answer to a Comment by Belonoshko [Phys. Rev. B 63, 096101 (2001)], we show that the B1-liquid melting curve of MgO obtained using two-phase simulations is in good agreement with the published one obtained using the Clausius-Clapeyron equation in conjunction with separate single phase calculations of liquid and solid

Studies of fullerenes and carbon nanotubes by an extended bond order potential

We present a novel approach to combine bond order potentials with long-range nonbond interactions. This extended bond order potential consistently takes into account bond terms and nonbond terms. It not only captures the advantages of the bond order potentials (i.e. simulating bond forming and breaking), but also systematically includes the nonbond contributions to energy and forces in studying the structure and dynamics of covalently bonded systems such as graphite, diamond, nanotubes, fullerenes and hydrocarbons, in their crystal and melt forms. Using this modified bond order potential, we studied the structure and thermal properties (including thermal conductivity) of C60 crystal, and the elastic properties and plastic deformation processes of the single-walled and double-walled nanotubes. This extended bond order potential enables us to simulate large deformations of a nanotube under tensile and compressive loads. The basic formulation in this paper is transferable to other bond order potentials and traditional valence force fields

Critical behavior in spallation failure of metals

Using molecular dynamics with an accurate many-body potential, we studied the rapid expansion of Ta metal following the high compression (50 to 100 GPa) induced by high velocity (2 to 4 km/s) impact. We find that catastrophic failure in this system coincides with a critical behavior characterized by a void distribution of the form N(v)∝V-τ, with τ∼2.2. This corresponds to a threshold in which percolation of the voids results in tensile failure. We define an order parameter (φ, the ratio of the volume of the largest void to the total void volume) which changes rapidly from ∼0 to ∼1 when the metal fails and scales with as φ∝(ρ-ρc)β with exponent β∼0.4, where ρ is the total void fraction. We found similar behavior for FCC Ni suggesting that this critical behavior is a universal characteristic for failure of solids in rapid expansion

The ferroelectric and cubic phases in BaTiO_3 ferroelectrics are also antiferroelectric

Using quantum mechanics (QM, Density Functional Theory) we show that all four phases of barium titanate (BaTiO3) have local Ti distortions toward (an octahedral face). The stable rhombohedral phase has all distortions in phase (ferroelectric, FE), whereas higher temperature phases have antiferroelectric coupling (AFE) in one, two, or three dimensions (orthorhombic, tetragonal, cubic). This FE–AFE model from QM explains such puzzling aspects of these systems as the allowed Raman excitation observed for the cubic phase, the distortions toward observed in the cubic phase using x-ray fine structure, the small transition entropies, the heavily damped soft phonon modes, and the strong diffuse x-ray scattering in all but the rhombohedral phase. In addition, we expect to see additional weak Bragg peaks at the face centers of the reciprocal lattice for the cubic phase. Similar FE–AFE descriptions are expected to occur for other FE materials. Accounting for this FE–AFE nature of these phases is expected to be important in accurately simulating the domain wall structures, energetics, and dynamics, which in turn may lead to the design of improved materials

Position of K atoms in doped single-walled carbon nanotube crystals

Recent experiments by Lee et al. [Nature (London) 363, 255 (1997)] show that doping carbon single-wall nanotube (SWNT) ropes with K, Rb, or Br2 leads to metallic conductivity, but the structure and properties are not known. We used molecular dynamics to predict structures and properties which should help motivate and interpret experiments on SWNT/K. We find the optimum stoichiometry to be KC16 if the K cannot penetrate the tubes and K1C10 ( K5exoK3endoC80, 3 within the tube) if they can. We predict the optimum structure and the associated powder-diffraction x-ray pattern expected for KnC80 from n = 0–10 (optimum is n = 5). The Young's modulus per tube along the tube axis varies from 640 to 525 GPa for n = 0 to 5

Morse stretch potential charge equilibrium force field for ceramics: Application to the quartz-stishovite phase transition and to silica glass

To predict phase transitions in ceramics and minerals from molecular dynamics simulations, we have developed a force field in which the charges are allowed to readjust instantaneously to the atomic configurations. These charges are calculated using the charge equilibration (QEq) method. In addition to electrostatics, a two-body Morse stretch potential is included to account for short-range nonelectrostatic interactions. This MS-Q potential is applied herein to SiO_2, where we find that it describes well the fourfold coordinated and sixfold coordinated systems (such as quartz and stishovite), silica glass, and the pressure-induced phase transition from quartz to stishovite

Phase diagram of MgO from density-functional theory and molecular-dynamics simulations

We use first-principles methods (no empirical parameters) to establish the phase diagram for the B1(NaCl), B2(CsCl), and liquid phases of MgO. We used density-functional theory with the generalized gradient approximation to predict the equation-of-state [volume versus pressure (V(P))] at 0 K for MgO in the low-density B1 (NaCl) phase and the high-density B2 (CsCl) phase. We find a pressure-induced phase transition at P=400 GPa. We then fitted an MS-Q type force field (FF) to the quantum results. This FF, denoted as qMS-Q FF, was then used in molecular dynamics (MD) simulations to investigate the phase coexistence curves of the B1-B2 and B1-liquid phases. This leads to a first-principles phase diagram for MgO for pressures up to 500 GPa and temperatures up to 8000 K. The accuracy of the fit of the qMS-Q FF to the quantum mechanics validates the functional form of the qMS-Q FF in which the charges are obtained from charge equilibration (QEq) and the nonelectrostatic forces are described with simple two-body Morse potentials. Such qMS-Q FF using no empirical data should be useful for MD or Monte Carlo simulations of many other materials

Energetics, structure, mechanical and vibrational properties of single-walled carbon nanotubes

In this paper, we present extensive molecular mechanics and molecular dynamics studies on the energy, structure, mechanical and vibrational properties of single-wall carbon nanotubes. In our study we employed an accurate interaction potential derived from quantum mechanics. We explored the stability domains of circular and collapsed cross section structures of armchair (n,n), zigzag(n,0) , and chiral (2n,n) isolated single-walled carbon nanotubes (SWNTs) up to a circular cross section radius of 170 Å. We have found three different stability regions based on circular cross section radius. Below 10 Å radius only the circular cross section tubules are stable. Between 10 and 30 Å both circular and collapsed forms are possible, however, the circular cross section SWNTs are energetically favorable. Beyond 30 Å (crossover radius) the collapsed form becomes favorable for all three types of SWNTs. We report the behavior of the SWNTs with radii close to the crossover radius ((45, 45), (80, 0), (70, 35)) under uniaxial compressive and tensile loads. Using classical thin-plane approximation and variation of strain energy as a function of curvature, we calculated the bending modulus of the SWNTs. The calculated bending moduli are [kappa][sub](n,n)=963.44 GPa, [kappa][sub](n,0)=911.64 GPa, and [kappa][sub](2n,n)=935.48 GPa. We also calculated the interlayer spacing between the opposite sides of the tubes and found d[sub](n,n)= 3.38 angstroms, d[sub](2n,n)= 3.39 angstroms, and d[sub](n,0)= 3.41 angstroms. Using an enthalpy optimization method, we have determined the crystal structure and Young's modulus of (10,10) armchair, (17,0) zigzag and (12, 6) chiral forms (which have similar diameter as (10,10)). They all pack in a triangular pattern in two dimensions. Calculated lattice parameters are a[sub](10,10)= 16.78 angstroms, a[sub](17,0)= 16.52 angstroms and a[sub](12,6)= 16.52 angstroms. Using the second derivatives of potential we calculated Young's modulus along the tube axis and found Y[sub](10,10)=640.30 GPa, Y[sub](17,0)=648.43 GPa, and Y[sub](12,6)=673.94 GPa. Using the optimized structures of (10,10), (12,6) and (17,0), we determined the vibrational modes and frequencies. Here, we report the highest in-plane mode, compression mode, breathing mode, shearing mode and relevant cyclop mode frequencies

Assessment of phenomenological models for viscosity of liquids based on nonequilibrium atomistic simulations of copper

The shear viscosity of liquid copper is studied using nonequilibrium molecular-dynamics simulations under planar shear flow conditions. We examined variation of viscosity as function of shear rate at a range of pressures (ca. 0 - 40 GPa). We analyzed these results using eight different phenomenological models and find that the observed non-Newtonian behavior is best described by the Powell-Eyring (PE) model: eta(gamma)=(eta(0)-eta(infinity))sinh(-1)(tau gamma)/(tau gamma)+eta(infinity), where gamma is the shear rate. Here eta(0) (the zero-shear-rate viscosity) extracted from the PE fit is in excellent agreement with available experimental data. The relaxation time tau from the PE fit describes the shear response to an applied stress. This provides the framework for interpreting the shear flow phenomena in complex systems, such as liquid metal and amorphous metal alloys