**A test of systematic coarse-graining of molecular dynamics simulations:
Transport properties**

CC Fu and PM Kulkarni and MS Shell and LG Leal, JOURNAL OF CHEMICAL PHYSICS, 139, 094107 (2013).

DOI: 10.1063/1.4819472

To what extent can a "bottom-up" mesoscale fluid model developed through
systematic coarse-graining techniques recover the physical properties of
a molecular scale system? In a previous paper ** C.-C. Fu, P. M.
Kulkarni, M. S. Shell, and L. G. Leal, J. Chem. Phys. 137, 164106
(2012)**, we addressed this question for thermodynamic properties through
the development of coarse-grained (CG) fluid models using modified
iterative Boltzmann inversion methods that reproduce correct pair
structure and pressure. In the present work we focus on the dynamic
behavior. Unlike the radial distribution function and the pressure,
dynamical properties such as the self-diffusion coefficient and
viscosity in a CG model cannot be matched during coarse-graining by
modifying the pair interaction. Instead, removed degrees of freedom
require a modification of the equations of motion to simulate their
implicit effects on dynamics. A simple but approximate approach is to
introduce a friction coefficient,gamma, and random forces for the
remaining degrees of freedom, in which case gamma becomes an additional
parameter in the coarse-grained model that can be tuned. We consider the
non-Galilean-invariant Langevin and the Galilean-invariant dissipative
particle dynamics (DPD) thermostats with CG systems in which we can
systematically tune the fraction phi of removed degrees of freedom.
Between these two choices, only DPD allows both the viscosity and
diffusivity to match a reference Lennard-Jones liquid with a single
value of gamma for each degree of coarse-graining phi. This friction
constant is robust to the pressure correction imposed on the effective
CG potential, increases approximately linearly with phi, and also
depends on the interaction cutoff length, r(cut), of the pair
interaction potential. Importantly, we show that the diffusion constant
and viscosity are constrained by a simple scaling law that leads to a
specific choice of DPD friction coefficient for a given degree of
coarse-graining. Moreover, we find that the pair interaction distance
cutoffs used for DPD random and dissipative forces should be considered
separately from that of the conservative interaction potential. (C) 2013
AIP Publishing LLC.

Return to Publications page