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[lammps-users] VACF like white nosie

# [lammps-users] VACF like white nosie

 From: "????" <1178408727@...1204...> Date: Sat, 19 May 2018 03:42:49 +0800

Dear Lammps user

when I use compute vacf to calculate the methane, the output is like white noise as shown in the picture.

"compute vacf  all  vacf 1 1 1
fix 3 all ave/time 10 10 100 c_vacf[4] file vacf.dat"

Here is the main script:
units real
variable       T equal 95
variable       Pressure equal 100
variable       V equal vol
variable       dt equal 0.01
variable       p equal 400 # correlation length
variable       s equal 1 # sample interval
variable       d equal \$p*\$s # dump interval
variable       steps  equal 500000

# convert from LAMMPS real units to SI

variable kB equal 1.3806504e-23 # [J/K] Boltzmann
variable kCal2J equal 4186.0/6.02214e23
variable A2m equal 1.0e-10
variable fs2s equal 1.0e-15
variable       atm2Pa equal 101325.0

variable convertK equal \${kCal2J}*\${kCal2J}/\${fs2s}/\${A2m}
variable convertV equal \${atm2Pa}*\${atm2Pa}*\${fs2s}*\${A2m}*\${A2m}*\${A2m}

boundary p p p
atom_style full

pair_style lj/cut/coul/cut 15
bond_style harmonic
angle_style harmonic
pair_modify mix geometric tail yes
special_bonds lj/coul 0.0 0.0 0.5
pair_coeff   1 1  0.066 3.5      # CT
pair_coeff   2 2  0.03  2.5      # HC
pair_coeff   1 2  0.044497191  3      # HC

# the remaining parameters are inferred from mixing.
bond_coeff   1   340.0   1.09    # CT-HC
angle_coeff  1    33.0   107.8     # HC-CT-HC
#bond_coeff   1   2845.12   1.09    # CT-HC
#angle_coeff  1    276.14 107.8     # HC-CT-HC

neighbor 1.5 bin
neigh_modify every 10 delay 20 check yes
fix     1 all npt temp \$T \$T 100 drag 0.2  iso \${Pressure} \${Pressure} 1000
thermo_style custom step temp pe ke press vol density
velocity all create \$T 432567 dist uniform
timestep       \${dt}
thermo         \$d
run   100000
reset_timestep 0

compute vacf  all  vacf 1 1 1
fix 3 all ave/time 10 10 100 c_vacf[4] file vacf.dat

run \${steps}