摘要:

We have developed a method for accelerating equilibration in numerical simulations of glassy models, using the Frenkel‐Kontorova model for developing and testing the algorithm. This simple model is like a glass in that relaxation times diverge rapidly as the critical temperature is reached, so that no matter how slowly it is cooled, the system eventually gets stuck in some random metastable configuration. By adding ‘‘numerical enzymes,’’ or long‐range Monte Carlo moves which precisely eliminate certain barriers to relaxation, we can equilibrate rapidly to significantly lower temperatures. Our numerical method for developing these enzymes is a learning algorithm motivated by Darwinian evolution.

ISSN:0094-243X    

DOI:10.1063/1.42451

出版商:AIP    

年代:1992

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.42452

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

To investigate dynamical aspects of crystallization, two nonlinear stochastic models are proposed based on the density functional theory. In addition to a first‐order freezing transition we observe a new dynamic transition where the symmetry of the models are broken.

ISSN:0094-243X    

DOI:10.1063/1.42453

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Simulations of glass transitions need to cope with exploding relaxation times especially in polymer systems which have ana priorislow dynamics. The longer the times your simulation method can explore, the closer you can approach the glass transition temperature. Therefore, we propose the Monte Carlo study of a polymer lattice model, namely the bondfluctuation model. The infinite temperature properties of this model have been extensively studied. Adding potentials that favor long bonds and/or increase the stiffness of the chains introduces a glass transition into the model. Using a mapping to Bisphenol‐A‐Polycarbonate we reproduce the Vogel‐Fulcher parameters experimentally found for the viscosity of BPA‐PC. The microscopic structural relaxation is analyzed according to mode–coupling predictions and is well comparable to experimental results found on Polybutadiene.

ISSN:0094-243X    

DOI:10.1063/1.42454

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We report on the molecular‐dynamics (MD) study of the binary soft‐sphere fluids, focusing our attention on their dynamical properties of the supercooled fluid phase and the dynamical anomalies associated with the liquid‐glass transition. The time dependent behavior of the mean‐square displacement, intermediate scattering functions, both of the self (incoherent) and total correlations, also the dynamical structure factor and non‐Gaussian parameter (NGP) are examined. Because of a rather short time window searched by the MD simulation, it is only an apparent dynamical behavior (e.g. on the diffusion coefficient, the long‐time decay of the density autocorrelation function and so on) that we can observe through the MD simulation in the vicinity of the transition, in which the longest relaxation time far exceeds the time scale of the MD simulation. Therefore, it accompanies severe difficulties in practice with an accurate determination of the glass transition temperature of the system from the properties of the long time behavior alone. The results of the MD simulation are compared with the trapping diffusion model (TDM) recently developed by Odagaki and Hiwatari. It is concluded that our results of the MD simulation are in excellent agreement with the TDM. With this, the glass transition can be more properly determined by maxima of the NGP, which take place in an intermediate time scale comparable to that of the MD simulation.

ISSN:0094-243X    

DOI:10.1063/1.42455

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Quantitative measures of the time required for effective ergodicity to be realized in a dynamical system are developed and applied to computer simulations of strongly supercooled Lennard‐Jones fluid mixtures. The techniques employed are generally applicable to the study of ergodic‐nonergodic transitions and can also be used in the design of long simulations.

ISSN:0094-243X    

DOI:10.1063/1.42456

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The main results from simulations of two different model systems, the soft‐sphere alloy and a polydisperse Yukawa fluid that has been studied with both Newtonian and Brownian dynamics, in the supercooled re´gime, are recalled. In particular it is emphasized that the only well established qualitative change resembling a ‘kinetic glass transision’ is a crossover in the microscopic self‐diffusion mechanism. One then briefly presents the similarities and differences with the predictions of mode‐coupling theory: although the agreement is fair for intermediate relaxation times, it seems that the critical laws are never clearly seen in numerical results, and that the theory provides a poor quantitative estimate of the thermodynamic coefficients of the glass.

ISSN:0094-243X    

DOI:10.1063/1.42433

出版商:AIP    

年代:1992

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.42434

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Molecular dynamics calculations are carried out for two Coulombic glasses, LiCl and LiCl‐KCl‐CsCl. Stillinger‐Weber’s inherent structures (the configurations of potential minima) which ions in glassy state undergo during 100 ps were explored for two systems. The ions in the LiCl‐KCl‐CsCl glass diffuse collectively via the successive potential basins whose minima are distributed along zigzag ways. On the contrary, in pure LiCl glass several ions transit to the next potential basins while the remaining ions continue to stay at almost the same potential basins.

ISSN:0094-243X    

DOI:10.1063/1.42435

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Molecular dynamics (MD) simulation of lithium metasilicate (Li2SiO3) has been performed to study the structural and dynamical characteristics near the glass transition temperature.The geometrical structure analysis reveals that the most probable numbers of ‘‘contact pairs (N)’’ of oxygen atoms satisfy a relation N=3V–6 (V=3 to 7) in a glassy state, where V is the co‐ordination number of oxygen around the lithium. On the other hand, in a liquid state, the most probable structure is represented by N<3V–6, which means an excess ‘free space’ to be available among oxygen atoms. These geometrical changes cause dynamical significance near the transition point.

ISSN:0094-243X    

DOI:10.1063/1.42436

出版商:AIP    

年代:1992

数据来源: AIP