Molecular dynamics of evaporative cooling of water clusters.

The cooling of water clusters through evaporation into a vacuum is studied using classical molecular dynamics with the SPC water model, and the results are compared with macroscopic theory. A simple model based on the Hertz-Knudsen equation significantly underestimates cooling rates. A modified approach that accounts for the Kelvin equation provides better results.

Novel barostat implementation for molecular dynamics.

We propose a novel implementation of the extended-dynamics equations for isothermal-isobaric ensemble in molecular dynamics, as the Martyna-Tobias-Klein thermostat and barostat. This method is suitable for systems with constraints and the Verlet-family integrators. Instead of iterations or the Trotter-expansion-based methods, both velocities and box sizes (scaling of bond lengths) are predicted.

Properties of water and argon clusters developed in supersonic expansions.

Using adiabatic molecular dynamics coupled with the fluid dynamics equations, we model nucleation in an expanding beam of water vapor and argon on a microsecond scale. The size distribution of clusters, their temperature, and pickup cross sections in dependence on velocity are investigated and compared to the geometric cross sections and the experiment. The clusters are warmer than the expanding gas because of the time scale of relaxation processes.

Does the Sign of Charge Affect the Surface Affinity of Simple Ions?

The role the charge sign of simple ions plays in determining their surface affinity in aqueous solutions is investigated by computer simulation methods. For this purpose, the free surface of aqueous solutions of fictitious salts is simulated at finite concentration both with nonpolarizable point-charge and polarizable Gaussian-charge potential models. The salts consist of monovalent cations and anions that are, apart from the sign of their charge, identical to each other.

Electron-induced fragmentation of water droplets: Simulation study.

The transport of free electrons in a water environment is still poorly understood. We show that additional insight can be brought about by investigating fragmentation patterns of finite-size particles upon electron impact ionization. We have developed a composite protocol aiming to simulate fragmentation of water clusters by electrons with kinetic energies in the range of up to 100 eV.

Molecular Dynamics of Heterogeneous Systems on GPUs and Their Application to Nucleation in Gas Expanding to a Vacuum.

Expansion of water vapor through a small orifice to a vacuum produces liquid or frozen clusters which in the experiment serve as model particles for atmospheric aerosols. Yet, there are controversies about the shape of these clusters, suggesting that the nucleation process is not fully understood. Such questions can be answered by molecular dynamics simulations; however, they require microsecond-scale runs with thousands of molecules and accurate energy conservation.

Single-Particle Dynamics at the Intrinsic Surface of Aqueous Alkali Halide Solutions.

The distribution of ions in the proximity of the liquid-vapor interface of their aqueous solution has been the subject of an intense debate during the last decade. The effects of ionic polarizability have been one of its salient aspects. Much less has been said about the corresponding dynamical properties, which are substantially unexplored.

Surface Affinity of Alkali and Halide Ions in Their Aqueous Solution: Insight from Intrinsic Density Analysis.

The surface tension of all aqueous alkali halide solutions is higher than that of pure water. According to the Gibbs adsorption equation, this indicates a net depletion of these ions in the interfacial region. However, simulations and experiments show that large, soft ions, such as I, can accumulate at the liquid/vapor interface.

Pressure in Molecular Simulations with Scaled Charges. 1. Ionic Systems.

Charge scaling, rationalized as MDEC (molecular dynamics in electronic continuum) or ECC (electronic continuum correction), has become a widely used simple approach to how to avoid self-consistent induced dipoles yet approximately take into account the effects of electronic polarizability. It has been assumed that the continuum permittivity does not depend on density; in turn, pressure is calculated by standard formulas. In this work, we elaborate a complementary approximation of density-independent molecular polarizability and derive formulas for pressure corrections within the MDEC framework; real behavior lies between these two extremes.

Free Energy of Classical Molecular Crystals by Thermodynamic Integration from a Harmonic Reference.

We develop an algorithm for calculating the normal modes of vibration of mechanical systems with constraints, particularly of molecules with rigid bonds and models of rigid molecules, and use it to obtain the harmonic free energy of a crystal. The anharmonic correction is then calculated by the conventional thermodynamic integration over temperature in the NVT ensemble. Attention is paid to finite-size errors, tail corrections, thermostat choice, ergodicity, and other sources of inaccuracies.

Chemical potentials of alkaline earth metal halide aqueous electrolytes and solubility of their hydrates by molecular simulation: Application to CaCl, antarcticite, and sinjarite.

We present a molecular-level simulation study of CaCl in water and crystalline hydrates formed by CaCl at ambient (298.15 K, 1 bar) conditions and at a high-temperature high-pressure state (365 K, 275 bars) typical of hydraulic fracturing conditions in natural-gas extraction, at which experimental properties are poorly characterized. We focus on simulations of chemical potentials in both solution and crystalline phases and on the salt solubility, the first time to our knowledge that such properties have been investigated by molecular simulation for divalent aqueous electrolytes.

Accurate Binding of Sodium and Calcium to a POPC Bilayer by Effective Inclusion of Electronic Polarization.

Binding affinities and stoichiometries of Na and Ca ions to phospholipid bilayers are of paramount significance in the properties and functionality of cellular membranes. Current estimates of binding affinities and stoichiometries of cations are, however, inconsistent due to limitations in the available experimental and computational methods. In this work, we improve the description of the binding details of Na and Ca ions to a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer by implicitly including electronic polarization as a mean field correction, known as the electronic continuum correction (ECC).

Direct Molecular Dynamics Simulation of Nucleation during Supersonic Expansion of Gas to a Vacuum.

We develop a methodology for direct molecular-level simulation of adiabatic expansion of gas through a small orifice to a vacuum. The gas attains supersonic speeds, cools, and nucleates. The proposed approach combines equations of frictionless fluid dynamics with molecular dynamics simulation in an expanding periodic box.

Solubility of NaCl in water and its melting point by molecular dynamics in the slab geometry and a new BK3-compatible force field.

Saturated concentration of rock salt in water is determined by a simulation of brine in contact with a crystal in the slab geometry. The NaCl crystals are rotated to expose facets with higher Miller indices than [001] to brine. The rock salt melting point is obtained by both the standard and adiabatic simulations in the slab geometry with attention paid to finite size effects as well as to a possible influence of facets with higher Miller indices and applied stress.

Residual entropy of ice III from Monte Carlo simulation.

We calculated the residual entropy of ice III as a function of the occupation probabilities of hydrogen positions α and β assuming equal energies of all configurations. To do this, a discrete ice model with Bjerrum defect energy penalty and harmonic terms to constrain the occupation probabilities was simulated by the Metropolis Monte Carlo method for a range of temperatures and sizes followed by thermodynamic integration and extrapolation to N = ∞. Similarly as for other ices, the residual entropies are slightly higher than the mean-field (no-loop) approximation.

Residual entropy of ices and clathrates from Monte Carlo simulation.

We calculated the residual entropy of ices (Ih, Ic, III, V, VI) and clathrates (I, II, H), assuming the same energy of all configurations satisfying the Bernal-Fowler ice rules. The Metropolis Monte Carlo simulations in the range of temperatures from infinity to a size-dependent threshold were followed by the thermodynamic integration. Convergence of the simulation and the finite-size effects were analyzed using the quasichemical approximation and the Debye-Hückel theory applied to the Bjerrum defects.

Static Dielectric Constant from Simulations Revisited: Fluctuations or External Field?

We compare two methods-the fluctuation formula and the application of an external electric field-for determining the static relative permittivity (static dielectric constant) from molecular simulations in the Ewald dielectric boundary conditions. We express both the systematic and statistical errors in terms of a dimensionless saturation (dielectric polarization reduced by its maximum) and show that both methods possess the same efficiency. Saturation in the fluctuation route depends on the number of particles and the permittivity of the surrounding medium, where a value of infinity (tinfoil) for the latter is usually reasonable but not necessarily optimum.

Time-Reversible Velocity Predictors for Verlet Integration with Velocity-Dependent Right-Hand Side.

Time-reversible velocity predictors (TRVPs) with increasing orders of the time-reversibility error are developed to be used with the Verlet integrator for equations of motion with the right-hand side depending on velocities. The method performs outside a possible SHAKE algorithm to constrain bond lengths and does not require repeated SHAKE iterations nor RATTLE. We have tested the TRVPs with the Nosé-Hoover thermostat on four model systems (coupled harmonic and anharmonic oscillators, liquid argon, SPC/E water, and a small peptide), comparing them to the Gear integrator with the Lagrangian formulation of constraint dynamics, the Martyna, Tuckerman, Tobias, and Klein (MTTK) method, and the velocity iteration method.

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.

Analytical expressions for the fourth virial coefficient of a hard-sphere mixture.

A method of numerical calculation of the fourth virial coefficients of the mixture of additive hard spheres is proposed. The results are compared with an exact analytical formula for the fourth partial virial coefficient B4[1] (i.e.

Virial coefficients and equation of state of the penetrable sphere model.

We study the penetrable sphere (alias square mound) model in the fluid phase by means of the virial expansion, molecular dynamics simulations, and Ornstein-Zernike integral equation. The virial coefficients up to B(8) are expressed as polynomials in the Boltzmann factor with the coefficients calculated by a Monte Carlo integration. New data for pressure and internal energy are obtained by molecular dynamics simulations with attention paid to finite-size errors and properties of the Andersen thermostat.

Aqueous solutions of ionic liquids: study of the solution/vapor interface using molecular dynamics simulations.

We performed a detailed molecular dynamics study of the interfacial structure of aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate in order to explain the anomalous dependence of the surface tension on concentration. At low concentrations the surface tension decreases with concentration. At higher concentrations the surface becomes saturated; a plateau is observed in simulations with a non-polarizable force field while a possible increase is detected in simulations with a polarizable force field.

Molecular simulations of aqueous electrolyte solubility: 1. The expanded-ensemble osmotic molecular dynamics method for the solution phase.

We have developed a molecular-level simulation technique called the expanded-ensemble osmotic molecular dynamics (EEOMD) method, for studying electrolyte solution systems. The EEOMD method performs simulations at a fixed number of solvent molecules, pressure, temperature, and overall electrolyte chemical potential. The method combines elements of constant pressure-constant temperature molecular dynamics and expanded-ensemble grand canonical Monte Carlo.

Nonanalytical equation of state of the hard sphere fluid.

A model based on classical nucleation theory is proposed to describe phase behavior in stable and metastable regions near a first-order phase transition. The resulting equation of state is not an analytical function at the phase transition point. The model is tested on the hard sphere fluid where it is combined with the virial expansion at low densities.

Gear formalism of the always stable predictor-corrector method for molecular dynamics of polarizable molecules.

The recently proposed always stable predictor-corrector method for molecular dynamics of polarizable molecules [J. Kolafa, J. Comput.