Relaxation dynamics and power spectra of liquid water: a molecular dynamics simulation study

We present molecular dynamics simulations of liquid water at normal and supercooled conditions. Autocorrelation functions (ACFs) of several structural quantities and their fourier transforms are obtained and analysed. Structural correlations and relaxation times increase linearly with degree of supercooling. Power spectra of ACFs show increase in librational motion of liquid water with cooling. These modes intensify with supercooling because of structuring and ordering of water molecules. Overall, liquid water structure is homogenous over the temperatures and pressures studied and undergoes fluctuation–dissipation in its local-density variations [English and Tse, Phys. Rev. Lett. 106, 037801 (2011)].     

Pressure-induced structural transition in amorphous GeO2: a molecular dynamics simulation

We studied the structural and dynamical properties of amorphous germanium dioxide (GeO₂) from low to high pressure by means of the classical molecular dynamics technique. The simulations were done in the micro-canonical ensemble, with systems at densities ranged from 3.16 to 6.79 g/cm³, using a pairwise potential. The network topology of the systems is analyzed at atomic level through partial pair correlations, coordination number and angular distributions. The dynamic properties were characterized by means of the vibrational density of states. According the density increases, a structural transformation from a short-range order, defined by a building block composed by a basic (GeO₄) tetrahedron, to a basic (GeO₆) octahedron is observed. The vibrational density of states also presents important changes when the density increases, with a low frequency band lessened, and a high density band wider and flatter.     

Thermodynamic properties of noble metal clusters:molecular dynamics simulation

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N ⩾ 312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting/freezing circle is derived in noble metal clusters.     

Thermodynamic properties of noble metal clusters: molecular dynamics simulation

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N ? 312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting/freezing circle is derived in noble metal clusters.     

Fragmentation of water clusters: Molecular-dynamics simulation study

The fragmentation of water clusters, [(H ₂ O)_n;n = 2-8], have been investigated by using molecular-dynamics simulation method. In the simulations a polarizable-dissociable potential energy function for water has been used. Particular attention has bee paid to investigate the effect of structural properties and cluster size on the fragmentation. Received 27 April 2000 and Received in final form 6 October 2000     

Hydration and translocation of an excess proton in water clusters: Anab initio molecular dynamics study

The hydration structure and translocation of an excess proton in hydrogen bonded water clusters of two different sizes are investigated by means of finite temperature quantum simulations. The simulations are performed by employing the method of Car-Parrinello molecular dynamics where the forces on the nuclei are obtained directly from ‘on the fly’ quantum electronic structure calculations. Since no predefined interaction potentials are used in this scheme, it is ideally suited to study proton translocation processes which proceed through breaking and formation of chemical bonds. The coordination number of the hydrated proton and the index of oxygen to which the excess proton is attached are calculated along the simulation trajectories for both the clusters.     

Conformational properties of cyclooctane: a molecular dynamics simulation study

Atomistic molecular dynamics simulations have been used to elucidate the conformational properties of cyclooctane in the gas and bulk liquid phases. Accurate reproduction of the gas phase structure, and of the liquid phase densities and solubility parameters have been used as prerequisites to the prediction of conformational properties. The gas phase results clearly indicate the presence of a conformational mixture consisting of the crown, boat-chair, twist-boat-chair and boat-boat conformers at all temperatures (161, 313 and 400 K) studied. The fraction of the crown family of conformers was found to be relatively insensitive to temperature. However, the relative concentrations of the twist-boat-chair and boat-chair conformations was found to be highly temperature dependent with the boat-chair being favoured at low temperatures. Bulk packing was found to have a profound effect on the conformational properties in the liquid phase. At the temperatures studied (313 and 400 K) the boat-chair family was predominant, with the crown and boat families being essentially absent. The twist-boat-chair conformation was detected in the liquid phase at both temperatures. The pseudorotation pathway for the twist-boat-chair to boat-chair interconversion was prevalent in both gas and liquid phases establishing the conformational flexibility and the relative importance of the twist-boat-chair conformer in comparison to the crown family. The study successfully explains the separate experimental findings in both the gas and liquid phases of cyclooctane.     

A study of cavitation nucleation in pure water using molecular dynamics simulation

To discover the microscopic mechanism responsible for cavitation nucleation in pure water, nucleation processes in pure water are simulated using the molecular dynamics method. Cavitation nucleation is generated by uniformly stretching the system under isothermal conditions, and the formation and development of cavitation nuclei are simulated and discussed at the molecular level. The processes of energy, pressure, and density are analyzed, and the tensile strength of the pure water and the critical volume of the bubble nuclei are investigated. The results show that critical states exist in the process of cavitation nucleation. In the critical state, the energy, density, and pressure of the system change abruptly, and a stable cavitation nucleus is produced if the energy barrier is broken and the critical volume is exceeded. System pressure and water density are the key factors in the generation of cavitation nuclei. When the critical state is surpassed, the liquid is completely ruptured, and the volume of the cavitation nucleus rapidly increases to larger than 100 nm³; at this point, the surface tension of the bubble dominates the cavitation nucleus, instead of intermolecular forces. The negative critical pressure for bubble nucleation is -198.6 MPa, the corresponding critical volume is 13.84 nm³, and the nucleation rate is 2.42×10³² m^-3·^-1 in pure water at 300 K. Temperature has a significant effect on nucleation: as the temperature rises, nucleation thresholds decrease, and cavitation nucleation occurs earlier.     

Pressure-induced phase transition of zinc-blende AlN: An ab initio molecular dynamics study

An ab initio constant pressure technique is carried out to study the pressure-induced phase transition of the zinc blende AlN (aluminum nitride). A first order phase transformation into a rock salt structure is observed in the constant pressure simulations. The transformation is accompanied by an initial tetragonal distortion and a subsequent shearing, similar to that found in the other zinc blende structured materials. This phase transition should occur around 6.2 GPa based upon the enthalpy calculations.     

Growth of clusters in a first-order phase transition

The results of computer simulations of phase separation kinetics in a binary alloy quenched from a high temperature are analyzed in detail, using the ideas of Lifshitz and Slyozov. The alloy was modeled by a three-dimensional Ising model with Kawasaki dynamics. The temperature after quenching was 0.59T _c, whereT _c is the critical temperature, and the concentration of minority atoms was=0.075, which is about five times their largest possible single-phase equilibrium concentration at that temperature. The time interval covered by our analysis goes from about 1000 to 6000 attempted interchanges per site. The size distribution of small clusters of minority atoms is fitted approximately byc ₁(1-)³ w(t),c ₁ (1–)⁴ Q _l w(t)^l(2l10); wherec _l is the concentration of clusters of sizel;Q ₂,...,Q ₁₀ are known constants, the cluster partition functions;t is the time; andw(t)=0.015(1+7.17t ^–1/3). The distribution of large clusters (l20) is fitted approximately by the type of distribution proposed by Lifshitz and Slyozov,c _l ,(t)=–(d/dl) [lnt+p (l/t)], where is a function given by those authors and is defined by(x)=C _o e–x-C ₁ e ^–4x/3-C ₂ e ^–5x/3;C ₀,C ₁,C ₂ are constants determined by considering how the total number of particles in large clusters changes with time.Supported by the U.S. Air Force Office of Scientific Research under Grant No. 78-3522 and by the U.S. Department of Energy under Contract No. EY-76-C-02-3077*000.     

Molecular dynamics simulation of the pressure-induced phase transition in BaFCl

The structural phase transition in BaFCl under high-pressure up to 30 v GPa has been studied using molecular dynamics (MD) method. It was found that BaFCl transforms from a tetragonal structure to a monoclinic structure in the upstroke process and then the tetragonal structure is recovered upon releasing the pressure. The atomistic mechanisms of the transformation have been examined using the pair-correlation functions and the coordination numbers for the lattices with or without vacancies in the MD cell. It was also demonstrated that the Cl atoms between the adjacent weakly bonded Cl layers shift in the compressed direction and move to positions with a 2-fold coordination number at a nearest-neighbour site after the transition.     

Structure,energetic and phase transition of small nickel-palladium heterogeneous clusters

Molecular dynamics simulation (MD) with Sutton-Chen potential for palladium-palladium, nickel-nickel and palladium-nickel interactions has been used to generate the minimum energy structures and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ni _n Pd_(13?n) for n ≤ 13. Thirteen particle icosahedral clusters of neat palladium and nickel atoms were first reproduced accordingly with the results in literature. Then in the palladium icosahedra, each palladium atom has been successively replaced by nickel atom. Calculation is repeated for both palladium-centered and nickel-centered clusters. It is found that the nickel-centered clusters are more stable than the palladium-centered clusters and cohesive energy increases along the palladium end to nickel end. Phase transition of each cluster from one end-species to the other end-species is studied by means of caloric curve, root mean square bond fluctuation and heat capacity. Trend in variation of melting temperature is opposite to the energy trend. Palladium-centered cluster shows a premelting at low temperature due to the solid-solid structural transition. Species-centric order parameters developed by Hewage and Amar is used to understand the dynamic behavior in the solid-solid transition of palladium-centered cluster to more stable nickel-centered cluster (premelting). This species-centric order parameter calculation further confirmed the stability of nickel-centered species over those of palladium-centered species and solid-solid structural transition at low temperature.     

From the bulk to clusters: Solid-liquid phase transitions and precursor effects

A review is given of the experiments done in order to understand the initial stages of melting and solidification of a material. We emphasize on new results obtained for ultrafine metal particles by using high sensitivity reflectance and electron microscopy measurements. These experiments show that melting certainly begins through a continuous and reversible transition and proceeds through the usual first-order transition. For ultrafine particles, these two processes have a relative importance which depend on the size, the first process having the major importance for the smallest particles. Undercooled state and subsequent solidification depend also on the size of the particle. For the smallest particles, the maximum supercooling temperature is found to coincide with the final temperature of the continuous transition leading to disappearance of the hysteretic behaviour usually found in solid-liquid transitions for ultrafine particles.     

Femtosecond laser ablation of metals:a molecular dynamics simulation study

Using molecular dynamics (MD) methods combining with two-step radiation heating model, the mechanisms of ablation and the thermodynamic states at Ni surface under femtosecond laser irradiation are investigated. Simulation results show that the main mechanisms of ablation are evaporation and tensile stresses generated inside the target. The velocity of stress wave is predicted to be nearly equal to sound velocity. The rates of ablation at different fluences obtained from simulations are in good agreement with experimental data. Superheating phenomenon is also discovered.     

Acetonitrile revisited: a molecular dynamics study of the liquid phase

The subject of this report is the calibration of a model of the liquid state of acetonitrile (methyl cyanide). The model describes the liquid in terms of molecular mechanics with each molecule of the liquid treated as a rigid body that is composed of three interaction sites, between which Coulomb and dispersion interactions are computed. A brief overview of the literature on such models is given and a set of parameters for the model is presented. The representation of liquid acetonitrile produced by the parameters is compared to that produced by several other parameter sets available in the literature. It is concluded that, of the parameter sets for the three-site molecular mechanics model that currently are available, under the simulation conditions used, the one presented produces the most rounded representation of the properties of liquid acetonitrile.     

Structural transition in small gas-like clusters

We propose a model, which is an alternative to the droplet model and presumes that the number of bonds between the atoms is a minimum, to describe highly excited clusters containing a small number of atoms. It is shown that at sufficiently high temperatures such a structure, which has the form of a system of spontaneously appearing chains of atoms (virtual chains),is realized with a greater probability than the close-packed structure. Analytic estimates are supported by the results of numerical molecular-dynamics simulations. Zh. éksp. Teor. Fiz. 113, 181–190 (January 1998)