tensiletest.lmp

################################################################################
# GRAPHENE: TENSILE TEST
################################################################################
#
# This is a lammps input file to simulate the stretching or compression of
# graphene. The output data can be used to calculate the Young's modulus and the
# breaking stress/strain. Buckling of a graphene sheet can be studied by
# compressing the sheet.
#
# When using this script, make sure that lammps is installed with the USER-REAXC
# and the MANYBODY packages.
#
# Date: 28/03/2016
# Author: Jeroen Mulkers
# Mail: jeroen.mulkers@uantwerpen.be
#
################################################################################


# UNITS
# 'real' units are needed when using reax/c. The default time step (1fs) for
# real units is used. Some useful conversion factors are defined.
units        real
timestep     1
variable     fpress  equal 0.000101325  # atm -> GPa
variable     fenergy equal 0.043        # Kcal/mole -> eV

# GRAPHENE SHEET
# The atom positions and the simulation box size will be read from a data file.
# Reactive force fields (reaxFF) are used to describe the interaction between
# the carbon atoms.(The pair_style reax is deprecated. so we use the modern
# reax/c implementation)
dimension    3
boundary     p p f
atom_style   charge
read_data    graphene.dat
pair_style   reax/c NULL checkqeq no
pair_coeff   * * ffield.reax C

# OUTPUT
# Calculate the average total, kinetic and potential energy per atom. Stress and
# strain are defined in x and y direction. When calculating the stress, we
# assume that the thickness of graphene is 3.35A.
variable     tmp     equal lx
variable     lx0     equal ${tmp}
variable     tmp     equal ly
variable     ly0     equal ${tmp}
variable     etotal  equal etotal/atoms*${fenergy} #eV/atom
variable     pe      equal     pe/atoms*${fenergy} #eV/atom
variable     ke      equal     ke/atoms*${fenergy} #eV/atom
variable     strainx equal (lx-${lx0})/${lx0}
variable     strainy equal (ly-${ly0})/${ly0}
variable     stressx equal -pxx*(lz/3.35)*${fpress} #GPa
variable     stressy equal -pyy*(lz/3.35)*${fpress} #GPa
thermo_style custom step time temp etotal press v_etotal v_pe v_ke &
             v_strainx v_stressx v_strainy v_stressy

# TEMPERATURE SETTINGS
variable     temp index 300
variable     seed index 1717
velocity     all create ${temp} ${seed} dist uniform

# EQUILIBRATION
# Relaxation of the system will be done in a NPT ensemble. The size of the
# simulation box can change in x and y direction to resolve stresses. The number
# of time steps should be large enough to relax the system in your final
# simulation but can be small for debugging purposes.
fix          fnpt all npt temp ${temp} ${temp} 10 x 0 0 500 y 0 0 500
thermo       1
run          1000

# DEFORMATION
# Rescale the box to simulate stretching (scale > 1) or compressing (scale<1)
# of the graphene sheet. Again, a NPT ensemble is used. But this time the size
# of the box can only change in the direction perpendicular to the direction of
# deformation. Change x and y to switch the deformation direction.
fix          boxdeform all deform 1 x scale 2 remap x
fix          fnpt all npt temp ${temp} ${temp} 10 y 0 0 500
dump         1 all atom 10 dmp.lammpstrj
thermo       1
run          10000