# Dresden 2003 – wissenschaftliches Programm

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# DY: Dynamik und Statistische Physik

## DY 20: Soft matter I

### DY 20.2: Vortrag

### Dienstag, 25. März 2003, 10:00–10:15, G\"OR/226

**Construction and test of thermostats and twirlers for molecular rotations** — •Siegfried Hess — Inst. f. Theoret. Physik, Techn. Univ. Berlin, 10623 Berlin

Time reversible thermostats (Gaussian isokinetic, Nosé-Hoover and
generalisations) have found widespread applications in molecular dynamics
computer
simulations for many particles. When applied to systems of few particles in
two or
three dimensions it is desireable to have an additional ‘stirring’ or
‘twirling’ mechanism
which affects the directions of the velocities and not just their
magnitudes as all standard thermostats do. In particular, a randomization
of the angular momentum is wanted.
This point is evident for the extendable dumbbell model studied
recently^{1,2}.
The construction of a class of dynamic twirlers is outlined. A special
choice for a computationally stable twirler is described by
ṙ=** p**+… and ṗ=

**+…+**

*F**c*

_{st}(

**·**

*F***/**

*p**T*)

**×**

*W***. The new dynamic variable**

*p***obeys the equation Ẇ=−(**

*W**c*

_{st}/

*T*)

**×**

*F***, where**

*p**T*is the temperature. The quantity

*c*

_{st}is the stirring coupling coefficient which has to be chosen appropriately. The ellipses … stand for terms associated with the standard thermostats and with the influence of a shear flow as treated by the SLLOD algorithm. Results are presented for the isotropic harmonic oscillator and an extendable dumbbell

^{2}. Furthermore, the influence of confining walls on the rotation are analysed for the dumbbell.

1) C. Aust, M. Kröger, S. Hess, Macromol.

**32**(1999) 5660;

**35**(2002) 8621.

2) S. Hess, G.P. Morriss, Physica D (2003) in press.