Multi-particle collision dynamics explained

Multi-particle collision dynamics (MPC), also known as stochastic rotation dynamics (SRD),^[1] is a particle-based mesoscale simulation technique for complex fluids which fully incorporates thermal fluctuations and hydrodynamic interactions.^[2] Coupling of embedded particles to the coarse-grained solvent is achieved through molecular dynamics.^[3]

Method of simulation

The solvent is modelled as a set of

N point particles of mass m with continuous coordinates \vec{r} _{i} and velocities \vec{v} _{i} . The simulation consists of streaming and collision steps.

During the streaming step, the coordinates of the particles are updated according to

\vec{r} _{i} (t + \delta t _{MPC} ) = \vec{r} _{i} (t) + \vec{v} _{i} (t) \delta t _{MPC}

where

\delta t _{MPC} is a chosen simulation time step which is typically much larger than a molecular dynamics time step.

After the streaming step, interactions between the solvent particles are modelled in the collision step. The particles are sorted into collision cells with a lateral size

a . Particle velocities within each cell are updated according to the collision rule\vec{v} _{i} → \vec{v} _{CMS} + \hat{ R } (\vec_ - \vec_)

where

\vec{v} _{CMS} is the centre of mass velocity of the particles in the collision cell and \hat{ R } is a rotation matrix. In two dimensions, \hat{ R } performs a rotation by an angle + \alpha or - \alpha with probability 1 / 2 . In three dimensions, the rotation is performed by an angle \alpha around a random rotation axis. The same rotation is applied for all particles within a given collision cell, but the direction (axis) of rotation is statistically independent both between all cells and for a given cell in time.

If the structure of the collision grid defined by the positions of the collision cells is fixed, Galilean invariance is violated. It is restored with the introduction of a random shift of the collision grid.^[4]

Explicit expressions for the diffusion coefficient and viscosity derived based on Green-Kubo relations are in excellent agreement with simulations.^{[5] [6]}

Simulation parameters

The set of parameters for the simulation of the solvent are:

• solvent particle mass

m

• average number of solvent particles per collision box

n _{s}

• lateral collision box size

a

• stochastic rotation angle

\alpha

• kT (energy)
• time step

\delta t _{MPC}

The simulation parameters define the solvent properties, such as

λ = \delta t _{MPC} \sqrt{kT/m} D = (kT\delta t _{MPC})/(2 m) [ (d n _{s})/(( 1- \cos(\alpha))(n _{s} -1+ \exp ^{- n _{s}} )) -1 ] \nu D _{T}

where

d is the dimensionality of the system.

A typical choice for normalisation is

a =1 , kT =1 , m =1 . To reproduce fluid-like behaviour, the remaining parameters may be fixed as \alpha = 130 ^{o} , n _{s} = 10 , \delta t _{MPC} \in [ 0 . 01 ; 0 . 1 ] .^[7]

Applications

MPC has become a notable tool in the simulations of many soft-matter systems, including

• colloid dynamics^{[8] [9]}
• polymer dynamics^{[10] [11]}
• vesicles^[12]
• active systems
• liquid crystals^[13]

Notes and References

1. Book: 10.1007/978-3-540-87706-6_1 . 0808.2157 . 221 . 1–87 . Gompper . G. . Ihle . T. . Kroll . D. M. . Winkler . R. G.. Advanced Computer Simulation Approaches for Soft Matter Sciences III . Multi-Particle Collision Dynamics: A Particle-Based Mesoscale Simulation Approach to the Hydrodynamics of Complex Fluids . 2009 . 2009acsa.book....1G . 978-3-540-87705-9 . 8433369 .
2. 10.1063/1.478857 . 110 . 17 . Mesoscopic model for solvent dynamics . 1999 . The Journal of Chemical Physics . 8605–8613 . Malevanets . Anatoly . Kapral . Raymond. 1999JChPh.110.8605M .
3. 10.1063/1.481289 . 112 . 16 . Solute molecular dynamics in a mesoscale solvent . 2000 . The Journal of Chemical Physics . 7260–7269 . Malevanets . Anatoly . Kapral . Raymond. 2000JChPh.112.7260M . 73679245 . free .
4. 10.1103/PhysRevE.67.066705 . 16241378 . 67 . 6 . 066705 . Stochastic rotation dynamics. I. Formalism, Galilean invariance, and Green-Kubo relations . 2003 . Physical Review E . Ihle . T. . Kroll . D. M.. 2003PhRvE..67f6705I .
5. 10.1103/PhysRevE.70.035701 . 15524580 . 70 . 3 . 035701 . Resummed Green-Kubo relations for a fluctuating fluid-particle model . 2004 . Physical Review E . Ihle . T. . Tüzel . E. . Kroll . D. M.. cond-mat/0404305 . 2004PhRvE..70c5701I . 11272882 .
6. 10.1103/PhysRevE.72.046707 . 16383567 . 72 . 4 . 046707 . Equilibrium calculation of transport coefficients for a fluid-particle model . 2005 . Physical Review E . Ihle . T. . Tüzel . E. . Kroll . D. M.. cond-mat/0505434 . 2005PhRvE..72d6707I . 14413944 .
7. http://kups.ub.uni-koeln.de/volltexte/2007/2007/pdf/elgeti.pdf J. Elgeti "Sperm and Cilia Dynamics" PhD thesis, Universität zu Köln (2006)
8. 10.1103/PhysRevLett.93.220601 . 2004PhRvL..93v0601P . 93 . 22 . 220601 . Hydrodynamic and Brownian Fluctuations in Sedimenting Suspensions . 2004 . Physical Review Letters . Padding . J. T. . Louis . A. A.. 15601076 . cond-mat/0409133 . 119504730 .
9. 10.1103/PhysRevE.74.021403 . 74 . 2 . 021403 . Shear viscosity of claylike colloids in computer simulations and experiments . 2006 . Physical Review E . Hecht . Martin . Harting . Jens . Bier . Markus . Reinshagen . Jörg . Herrmann . Hans J.. 17025421 . cond-mat/0601413 . 2006PhRvE..74b1403H . 19998245 .
10. 10.1063/1.2041527 . 123 . 14 . Dynamics of polymers in a particle-based mesoscopic solvent . 2005 . The Journal of Chemical Physics . 144905 . Mussawisade . K. . Ripoll . M. . Winkler . R. G. . Gompper . G.. 16238422 . 2005JChPh.123n4905M .
11. 10.1140/epje/i2006-10220-0 . 17712520 . 23 . 4 . Hydrodynamic screening of star polymers in shear flow . 2007 . The European Physical Journal E . 349–354 . Ripoll . M. . Winkler . R. G. . Gompper . G.. 2007EPJE...23..349R . 36780360 .
12. 10.1103/PhysRevE.72.011901 . 16089995 . 72 . 1 . 011901 . Dynamics of fluid vesicles in shear flow: Effect of membrane viscosity and thermal fluctuations . 2005 . Physical Review E . Noguchi . Hiroshi . Gompper . Gerhard. 2005PhRvE..72a1901N .
13. K.-W. Lee and Marco G. Mazza. Stochastic rotation dynamics for nematic liquid crystals. Journal of Chemical Physics. 142 . 16. 10.1063/1.4919310. 2015. 164110. 25933755 . 1502.03293. 2015JChPh.142p4110L. 36839435 .