Greetings,
I was looking at the example for calculating the viscosity in 2D using the Green-Kubo method that I saw here: https://github.com/lammps/lammps/blob/master/examples/VISCOSITY/in.gk.2d. I have copied it below as well.
But for 2D you only have one off-diagonal component, P_xy, so the example code below somewhat confuses me. Particularly the two lines:
variable pxx equal pxx-press
and
variable eta equal 0.5*({etaxy}+{etaxx})
My apologies in advanced if I am missing something simple, and thanks for any advice one may have,
–Joe
sample LAMMPS input script for viscosity of 2d LJ liquid
Green-Kubo method via fix ave/correlate
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
Green-Kubo viscosity calculation
reset_timestep 0
Define distinct components of symmetric traceless stress tensor
variable pxy equal pxy
variable pxx equal pxx-press
fix SS all ave/correlate $s $p $d &
v_pxy v_pxx type auto file profile.gk.2d ave running
Diagonal components of SS are larger by factor 2-2/d,
which is 4/3 for d=3, but 1 for d=2.
See Daivis and Evans, J.Chem.Phys, 100, 541-547 (1994)
variable scale equal 1.0/$tvols*dt
variable diagfac equal 2-2/2
variable vxy equal trap(f_SS[3])*{scale}
variable vxx equal trap(f_SS[4])*{scale}/{diagfac}
thermo_style custom step temp 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}”