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lammps/examples/VISCOSITY/in.einstein.2d
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in.einstein.2d
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# sample LAMMPS input script for viscosity of 2d LJ liquid
# Einstein form of Green-Kubo
# settings
variable x equal 20
variable y equal 20
variable rho equal 0.6
variable t equal 1.0
variable rc equal 2.5
variable p equal 400 # correlation length
variable s equal 5 # sample interval
variable d equal $p*$s # dump interval
# problem setup
units lj
dimension 2
atom_style atomic
neigh_modify delay 0 every 1
lattice sq2 ${rho}
region simbox block 0 $x 0 $y -0.1 0.1
create_box 1 simbox
create_atoms 1 box
pair_style lj/cut ${rc}
pair_coeff * * 1 1
mass * 1.0
velocity all create $t 97287
# equilibration run
fix 1 all nve
fix 2 all langevin $t $t 0.1 498094
fix 3 all enforce2d
thermo $d
run 10000
velocity all scale $t
unfix 2
# Einstein viscosity calculation
reset_timestep 0
# Define distinct components of symmetric traceless stress tensor
variable pxy equal pxy
variable pxx equal pxx-press
fix avstress all ave/time $s $p $d v_pxy v_pxx ave one file einstein.dat
# Diagonal components of SS are larger by factor 2-2/d,
# which is 4/3 for d=3, but 1 for d=2.
# See Daivis and Evans, J.Chem.Phys, 100, 541-547 (1994)
variable scale equal vol/(2.0*$t*dt*$d)
variable diagfac equal 2-2/2
variable deltasqxy equal (f_avstress[1]*$d*dt)^2
variable deltasqxx equal (f_avstress[2]*$d*dt)^2/${diagfac}
# compute mean square displacements as running averages
fix avdeltasq all ave/time $d 1 $d v_deltasqxy v_deltasqxx ave running
# convert to viscosities
variable vxy equal f_avdeltasq[1]*${scale}
variable vxx equal f_avdeltasq[2]*${scale}
thermo_style custom step temp pe press pxy v_vxy v_vxx
run 500000
variable etaxy equal v_vxy
variable etaxx equal v_vxx
variable eta equal 0.5*(${etaxy}+${etaxx})
print "running average viscosity: ${eta}"
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