21,295 research outputs found
Thermodynamic scaling of diffusion in supercooled Lennard-Jones liquids
The manner in which the intermolecular potential u(r) governs structural
relaxation in liquids is a long standing problem in condensed matter physics.
Herein we show that diffusion coefficients for simulated Lennard-Jones m-6
liquids (8<m<36) in normal and moderately supercooled states are a unique
function of the variable rho^g/T, where rho is density and T is temperature.
The scaling exponent g is a material specific constant whose magnitude is
related to the steepness of the repulsive part of u(r), evaluated around the
distance of closest approach between particles probed in the supercooled
regime. Approximations of u(r) in terms of inverse power laws are also
discussed.Comment: 4 pages, 3 figure
Density Scaling and Dynamic Correlations in Viscous Liquids
We use a recently proposed method [Berthier L.; Biroli G.; Bouchaud J.P.;
Cipelletti L.; El Masri D.; L'Hote D.; Ladieu F.; Pierno M. Science 2005, 310,
1797.] to obtain an approximation to the 4-point dynamic correlation function
from derivatives of the linear dielectric response function. For four liquids
over a range of pressures, we find that the number of dynamically correlated
molecules, Nc, depends only on the magnitude of the relaxation time,
independently of temperature and pressure. This result is consistent with the
invariance of the shape of the relaxation dispersion at constant relaxation
time and the density scaling property of the relaxation times, and implies that
Nc also conforms to the same scaling behavior. For propylene carbonate and
salol Nc becomes constant with approach to the Arrhenius regime, consistent
with the value of unity expected for intermolecularly non-cooperative
relaxation.Comment: revisio
Molecular dynamics simulations of the Johari-Goldstein relaxation in a molecular liquid
Molecular dynamics simulations (mds) were carried out to investigate the
reorientational motion of a rigid (fixed bond length), asymmetric diatomic
molecule in the liquid and glassy states. In the latter the molecule reorients
via large-angle jumps, which we identify with the Johari-Goldstein (JG)
dynamics. This relaxation process has a broad distribution of relaxation times,
and at least deeply in the glass state, the mobility of a given molecule
remains fixed over time; that is, there is no dynamic exchange among molecules.
Interestingly, the JG relaxation time for a molecule does not depend on the
local density, although the non-ergodicity factor is weakly correlated with the
packing efficiency of neighboring molecules. In the liquid state the frequency
of the JG process increases significantly, eventually subsuming the slower
alpha-relaxation. This evolution of the JG-motion into structural relaxation
underlies the correlation of many properties of the JG- and alpha-dynamics.Comment: 12 pages, 6 figure
Rural and Small Town Employment: Structure by Industry
Community/Rural/Urban Development, Labor and Human Capital,
Adaptively Biased Molecular Dynamics for Free Energy Calculations
We present an Adaptively Biased Molecular Dynamics (ABMD) method for the
computation of the free energy surface of a reaction coordinate using
non-equilibrium dynamics. The ABMD method belongs to the general category of
umbrella sampling methods with an evolving biasing potential, and is inspired
by the metadynamics method. The ABMD method has several useful features,
including a small number of control parameters, and an numerical cost
with molecular dynamics time . The ABMD method naturally allows for
extensions based on multiple walkers and replica exchange, where different
replicas can have different temperatures and/or collective variables. This is
beneficial not only in terms of the speed and accuracy of a calculation, but
also in terms of the amount of useful information that may be obtained from a
given simulation. The workings of the ABMD method are illustrated via a study
of the folding of the Ace-GGPGGG-Nme peptide in a gaseous and solvated
environment.Comment: Revised version to appear in Journal of Chemical Physic
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