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MD-Simulation of Viscous Toluene. Ulf R. Pedersen & Thomas Schrøder. Department of Mathematics and Physics (IMFUFA), DNRF centre ”Glass and Time”, Roskilde University, Postbox 260, DK-4000 Roskilde, Denmark. Outline. Toluene like model. Molecular Dynamics are found using Newtonian mechanics.

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md simulation of viscous toluene

MD-Simulation of Viscous Toluene

Ulf R. Pedersen & Thomas Schrøder

Department of Mathematics and Physics (IMFUFA),DNRF centre ”Glass and Time”,Roskilde University, Postbox 260,DK-4000 Roskilde, Denmark

toluene like model
Toluene like model
  • Molecular Dynamics are found using Newtonian mechanics.
  • Here, forces are given by Lennard-Jones potentials.

Chemical structure

of toluene

A simple 1-component system

that does not crystallize:

Type A: OPLS-UA CH3 group

Type B: Benzene from the

Lewis-Wahnström OTP model

500 ns/day using

512 molecules on 4 processors

The Lennard-Jones potential

OPLS-UA: J. A. Chem Soc. 1984, vol. 106, p. 6638-6646

LW: Phys. Rev. E, 1994, vol, 50, num. 5, p. 3865-3877

structure
Structure

g(r), radial distribution function

A: methyl

B: benzene

~0.40 nm

~0.55 nm

~0.73 nm

the density during a cooling ramp
The density during a cooling ramp

Transition from liquid to solid on the simulated timescale

Tm: Melting temperature

Tc: Critical temperature where hopping accurse in dynamics

Tg: Glass transition temperature (t = 100 s)

Cooling rate: 37.5 K/ns

mean square displacement
Mean Square Displacement

Diffusion constant

140K

van hove correlation function at high temperature
Van Hove correlation function at high temperature

Hopping of methyl

Hopping of benzene/CM ?

4pr2Gs(r,t)

4pr2Gs(r,t)

van hove correlation function at low temperature
Van Hove correlation function at low temperature

Hopping of methyl

Hopping of benzene/CM ?

4pr2Gs(r,t)

4pr2Gs(r,t)

two aspects of the dynamics diffusion and rotation
Two aspects of the dynamics, diffusion and rotation

Non-exponential relaxation!

Intermediate scattering function

Dipole-dipole correlation

Fit to stretch exponentals are shown, f(t)=A exp(-(t/t)g).

t is a characteristic time, and g is the stretch

characteristic time and stretching exponents
Characteristic time and stretching exponents

Non-Arrhenius relaxation!

140K (hopping)

Characteristic times do not follow

an Arrhenius law, t(T) = t0exp(Ea/kbT)

Relaxation becomes more stretch

with decreasing temperature

relaxation in time and frequency domain
Relaxation in time and frequency domain

Prigogine-Defay ratio

and the one-parameter hypothesis

130 K

future work
Future work
  • One-parameter hypothesis (Prigogine-Defay ratio)
  • Compare dynamics between idealized model and more realistic model
  • Finite size effects?

notthe end …

slide14
Mish

The -process

movie
Movie

Center of mass

Methyl

Quench dynamics at 120 K, 1 sek ~ 0.7 ns

slide19

UA-OPLS

25 ns/day