My project is thermal conductivity in nano- ribbon graphene .I have a problem, I do not know for a nano- ribbon graphene with a length of 12nm and width of 8nm,What number should be p ? what is the numerical relationship between the length of graphene and p?

#SAMPLE lammps input script for thermal conductivity of GNR via the GK formalism

units metal

variable T equal 300

variable V equal vol

variable p equal 100 # correlation length

variable s equal 2 # sample interval

variable d equal $p*$s # dump interval

variable dt equal 0.001

# convert from LAMMPS real units to SI

variable kB equal 1.3806504e-23 # [J/K] Boltzmann

variable ev2J equal 1.60217653e-19

variable A2m equal 1.0e-10

variable ps2s equal 1.0e-12

variable convert equal {ev2J}*{ev2J}/{ps2s}/{A2m}

# setup problem

dimension 3

atom_style atomic

boundary p p p

read_data data.box

region box block -100 100 -50 250 -30 30 units box

mass 1 12.000

pair_style tersoff

pair_coeff * * sic.tersoff C

timestep 0.001

thermo $d

# equilibration and thermalization

velocity all create $T 102486 mom yes rot yes dist gaussian

fix NVT all nvt temp $T $T 10 drag 0.2

run 8000

# thermal conductivity calculation, switch to NVE if desired

unfix NVT

fix NVE all nve

reset_timestep 0

compute myKE all ke/atom

compute myPE all pe/atom

compute myStress all stress/atom virial

compute flux all heat/flux myKE myPE myStress

variable Jx equal c_flux[1]/vol

variable Jy equal c_flux[2]/vol

variable Jz equal c_flux[3]/vol

fix JJ all ave/correlate $s $p $d &

c_flux[1] c_flux[2] c_flux[3] type auto file J0Jt.dat ave running

variable scale equal {convert}/{kB}/$T/$T/$V*s*{dt}

variable k11 equal trap(f_JJ[3])*{scale}
variable k22 equal trap(f_JJ[4])*{scale}
variable k33 equal trap(f_JJ[5])*${scale}

thermo_style custom step temp v_Jx v_Jy v_Jz v_k11 v_k22 v_k33

run 50000

variable k equal (v_k11+v_k22+v_k33)/3.0

variable ndens equal count(all)/vol

print “average conductivity: $k[W/mK] @ T k, {ndens} /A^3”