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

read_data methane0.27.data

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}

Any useful advice is appreciate.

best ragars!

cheng