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Richard Berger
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#===========================================================================#
# polymer test #
# #
# Run consists of a lone 32-bead coarse-grained polymer #
# undergoing Brownian motion in thermal lattice-Boltzmann fluid. #
# #
# Here, gamma (used in the calculation of the monomer-fluid interaction #
# force) is set by the user (gamma = 0.03 for this simulation...this #
# value has been calibrated a priori through simulations of the drag #
# force acting on a single particle of the same radius). #
# Sample output from this run can be found in the file: #
# 'dump.polymer.lammpstrj' #
# and viewed using, e.g., the VMD software. #
# #
#===========================================================================#
units nano
dimension 3
boundary p p p
atom_style hybrid molecular
special_bonds fene
read_data data.polymer
#----------------------------------------------------------------------------
# Need a neighbor bin size smaller than the lattice-Boltzmann grid spacing
# to ensure that the particles belonging to a given processor remain inside
# that processors lattice-Boltzmann grid.
#----------------------------------------------------------------------------
neighbor 0.5 bin
neigh_modify delay 0 every 1 check yes
neigh_modify exclude type 2 2
neigh_modify exclude type 2 1
#----------------------------------------------------------------------------
# Implement a hard-sphere interaction between the particles at the center of
# each monomer (use a truncated and shifted Lennard-Jones potential).
#----------------------------------------------------------------------------
bond_style fene
bond_coeff 1 60.0 2.25 4.14195 1.5
pair_style lj/cut 1.68369
pair_coeff 1 1 4.14195 1.5 1.68369
pair_coeff 1 2 4.14195 1.5 1.68369
pair_coeff 2 2 0 1.0
mass * 0.000000771064
timestep 0.00003
#----------------------------------------------------------------------------
# ForceAtoms are the particles at the center of each monomer which
# do not interact with the fluid, but are used to implement the hard-sphere
# interactions.
# FluidAtoms are the particles representing the surface of the monomer
# which do interact with the fluid. Monomer surface is shell of radius 0.7
#----------------------------------------------------------------------------
group ForceAtoms type 1
group FluidAtoms type 2
#---------------------------------------------------------------------------
# Create a lattice-Boltzmann fluid covering the simulation domain.
# This fluid feels a force due to the particles specified through FluidAtoms
# (however, this fix does not explicitly apply a force back on to these
# particles. This is accomplished through the use of the lb/viscous
# fix).
# Uses the standard LB integration scheme, fluid viscosity = 0.023333333,
# fluid density= 0.0000166368, lattice spacing dx=1.0, and mass unit,
# dm=0.0000166368.
# Use the default method to calculate the interaction force between the
# particles and the fluid. This calculation requires the surface area
# of the composite object represented by each particle node. By default
# this area is assumed equal to dx*dx; however, since this is not the case
# here, it is input through the setArea keyword (i.e. particles of type 2
# correspond to a surface area of 0.2025=4 Pi R^2/N ).
# Use the trilinear interpolation stencil to distribute the force from
# a given particle onto the fluid mesh (results in a smaller hydrodynamic
# radius than if the Peskin stencil is used).
# Use a thermal lattice-Boltzmann fluid (temperature 300K, random number
# seed=15003). This enables the particles to undergo Brownian motion in
# the fluid.
#----------------------------------------------------------------------------
fix 1 FluidAtoms lb/fluid 3 1 0.023333333 0.0000166368 setArea 2 0.20525 dx 1.0 dm 0.0000166368 noise 300.0 15003
#----------------------------------------------------------------------------
# Apply the force from the fluid to the particles, and integrate their
# motion, constraining them to move and rotate together as a single rigid
# spherical object.
# Since both the ForceAtoms (central atoms), and the FluidAtoms (spherical
# shell) should move and rotate together, this fix is applied to all of
# the atoms in the system. However, since the central atoms should not
# feel a force due to the fluid, they are excluded from the fluid force
# calculation.
#----------------------------------------------------------------------------
fix 2 FluidAtoms lb/viscous
fix 3 all rigid molecule
#----------------------------------------------------------------------------
# To ensure that numerical errors do not lead to a buildup of momentum in the
# system, the momentum_lb fix is used every 10000 timesteps to zero out the
# total (particle plus fluid) momentum in the system.
#----------------------------------------------------------------------------
fix 4 all lb/momentum 10000 linear 1 1 1
#----------------------------------------------------------------------------
# Write position and velocity coordinates into a file every 2000 time steps.
#----------------------------------------------------------------------------
dump 1 ForceAtoms custom 2000 dump.polymer_default_gamma.lammpstrj id x y z vx vy vz
run 2000001