1. 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

2. Outline

3. 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 CH 3 group Type B: Benzene from the Lewis-Wahnström OTP model 500 ns/day using 512 molecules on 4 processors OPLS-UA: J. A. Chem Soc. 1984, vol. 106, p. 6638-6646 LW: Phys. Rev. E, 1994, vol, 50, num. 5, p. 3865-3877 The Lennard-Jones potential

4. Structure g(r), radial distribution function ~0.73 nm ~0.55 nm ~0.40 nm A: methyl B: benzene AB m [au]15.03577.106  [kJ/mol] 0.669445.72600  [nm] 0.39100.4963

5. The density during a cooling ramp Cooling rate: 37.5 K/ns Transition from liquid to solid on the simulated timescale T m : Melting temperature T c : Critical temperature where hopping accurse in dynamics T g : Glass transition temperature (  = 100 s)

6. Mean Square Displacement 140K Diffusion constant

7. Van Hove correlation function at high temperature Hopping of benzene/CM ? 4  r 2 G s (r,t) Hopping of methyl

8. Van Hove correlation function at low temperature Hopping of benzene/CM ? 4  r 2 G s (r,t) Hopping of methyl

9. Two aspects of the dynamics, diffusion and rotation Dipole-dipole correlationIntermediate scattering function Fit to stretch exponentals are shown, f(t)=A exp(-(t/  )  ).  is a characteristic time, and  is the stretch Non-exponential relaxation!

10. Characteristic time and stretching exponents 140K (hopping) Relaxation becomes more stretch with decreasing temperature Characteristic times do not follow an Arrhenius law,  (T) =  0 exp(E a /k b T) Non-Arrhenius relaxation!

11. Relaxation in time and frequency domain Prigogine-Defay ratio and the one-parameter hypothesis 130 K

12. Conclution

13. Future work One-parameter hypothesis (Prigogine-Defay ratio) Compare dynamics between idealized model and more realistic model Finite size effects? not the end …

14. Mish The  -process

15. Movie Quench dynamics at 120 K, 1 sek ~ 0.7 ns Center of massMethyl

16. MSD and diffusion

17. Mean Square Displacement 140K

18. Motivation Why do molecular dynamics simulation? Between experiment and theory: enables one to investigate theory in a controlled environment study microscopic details of the dynamics But: a simulation is based on a more or less realistic model limits in time and length scale

20. UA-OPLS 25 ns/day