István Borzsák

Shear viscosity of a simple fluid over a wide range of strain rates

The shear viscosity of the Weeks—Chandler—Andersen (WCA) fluid at the Lennard-Jones triple point has been calculated over a wide range of strain rates using the transient time correlation function (TTCF) formalism. It has been demonstrated that these calculations can be carried out at arbitrarily low strain rates with the precision of the Green—Kubo calculations. At high strain rates, the calculated data agree within the error bars with more precise data acquired using the computationally less demanding steady state (SS) non-equilibrium molecular dynamics (NEMD) method. The linear variation of viscosity with the square root of the strain rate is discussed.

On the convergence of Green's entropy expansion

Abstract We tested the convergence of Greens entropy expansion for hard sphere systems of different dimensions (D = 1, 2, 3). We compared the two-body and (for D= 1 and 3) the sum of the two-body and three-body entropy contributions to the total excess entropy in these systems. The qualitative properties of the expansion are very similar for the three tested models. The two-body contribution gives a good estimate of the excess entropy at low densities but underestimates the absolute value of the excess entropy at intermediate densities. In this region the inclusion of the three-body entropy contribution improves the accuracy of the expansion. Close to the phase transition density (in D= 2,3) the two-body entropy — due to fortuitous cancellations — is again a good approximation of the excess entropy.

Molecular dynamics simulation of ice XII

Abstract Molecular dynamics simulations have been performed on the newly discovered metastable ice XII. This new crystalline ice phase [C. Lobban, J.L. Finney, W.F. Kuhs, Nature (London) 391 (1998) 268] is proton-disordered. Thus 90 possible configurations of the unit cell can be constructed which differ only in the orientations of the water molecules. The simulation used the TIP4P potential model for water at constant temperature and density. About one-quarter of the initial configurations did not melt in the course of the simulation. This result is supportive of the experimental structure and also demonstrates the ability of this water model to study ice phases.

Correlation regimes in systems of soft spherical particles studied by their Lyapunov exponents

We performed molecular dynamics simulations of soft spherical particles over wide ranges of densities and temperatures corresponding to fluids, glasses and crystalline solids, and calculated the full Lyapunov spectra of these systems. For either phase corresponding exponents essentially scale with the square-root of the temperature in accordance with kinetic theory. The density dependence is more pronounced and less systematic. The shape of the spectrum of a glass is different from that of a crystalline solid at the same density and temperature and resembles the spectrum of the initial dense liquid-like phase. For dilute gases the sum of the positive exponents approaches zero and is increasingly dominated by the largest exponent. Although systematic changes of the Lyapunov spectra were observed, it seems that the spectral shape does not uniquely determine the phase of the system.

Effect of oscillatory shear on the fluid–solid phase transition of supercooled water

The present study aims at the methodological improvement of the electrofreezing of supercooled water by applying oscillatory shear along with a homogeneous electric field in non-equilibrium molecular dynamics simulations of simple water models (such as TIP4P and SPC/E). The application of a planar Couette flow field accounts for a moderate speedup in the phase transition. Since the system goes through a nonergodic glassy state in the course of the process, it is not surprising that a macroscopic manifestation of the Lyapunov instability inherent in the equations of motion is observed. The formation of a conjectured high-density ice polymorph (ice XII) has been studied in terms of the applied electric and oscillatory flow fields. The threshold value of the electric field for the crystallization has been determined. The method of applying the oscillatory flow field was also investigated in detail.