Hello all Dear lammps useres

I’m facing an odd problem with my simulation. I’m modeling an infinite CNT and another modified-functionalized CNT that are co-centered , thus the outer CNT is moving on the inner one with smaller diameter. the problem is no matter how many time steps I eliminate my infinite cnt center of mass momentum using fix momentum command, it moves! I managed to solve this problem by fix recenter but I’m not sure why this happened in fix momentum command, I’ve checked the control parameters and log file and every thing seems normal.

by the way I’ve used OPLS forcefield so I defined the extra angles and bonds and dihedrals which across the boundaries in data file.

Thanks for your kind help

this is my input file:

#processors 1 1 10

units real

dimension 3

boundary p p p

atom_style full

read_data pucnthimproper.data extra/atom/types 1 extra/bond/types 1 extra/angle/types 1 extra/dihedral/types 1

read_data innerperiodic.data add append offset 2 2 2 3 1

neighbor 2.0 bin

neigh_modify delay 2 every 1

region bottom block INF INF INF INF INF 5

region top block INF INF INF INF 355 INF

group pucnt type 1 2

group nanotube type 3

group cm id 536

#********************************************************** force-field *********************************************

pair_style lj/cut/coul/cut 10 10

bond_style harmonic

angle_style harmonic

dihedral_style opls

improper_style harmonic

special_bonds lj/coul 0.0 0.0 0.5

################set parameters#####################

## argon and PUCNTH opls

pair_coeff 1 1 0.070 3.55000

pair_coeff 2 2 0.030000 2.4200

pair_coeff 3 3 0.070 3.55000 # carbon 2 = nanotube inner

pair_modify mix geometric

bond_coeff 1 469.00 1.4000

bond_coeff 2 367.00 1.0800

bond_coeff 3 469.00 1.4000

angle_coeff 1 63.00 120.00

angle_coeff 2 35.00 120.00

angle_coeff 3 63.00 120.00

dihedral_coeff 1 0.0 7.250 0.0 0.0

dihedral_coeff 2 0.0 7.250 0.0 0.0

dihedral_coeff 3 0.0 7.250 0.0 0.0

dihedral_coeff 4 0.0 7.250 0.0 0.0

improper_coeff 1 0.55 180

################simualtion#####################

#kspace_style pppm 1e-4

compute com pucnt com

timestep 1

minimize 0.001 0.001 1000 10000

velocity pucnt create 300 1234 dist gaussian

velocity nanotube create 300 1234 dist gaussian

#delete_atoms overlap 1 argon nanotube mol yes

fix 1 nanotube nvt temp 300 300 100

fix 2 pucnt nve

compute 1 pucnt chunk/atom molecule

compute msd pucnt msd/chunk 1

fix 3 pucnt ave/time 1000 1 1000 c_msd file msd.out mode vector

dump t all xyz 1000 dumpt.xyz

thermo_style custom step temp press ke pe etotal c_com[1] c_com[2] c_com[3]

thermo 1000

fix momcnt nanotube momentum 100 linear 0 0 0 angular

run 1000000

undump t

write_data cocenterrealxed.data

dump 1 all xyz 1000 dump.xyz

dump 2 pucnt xyz 1000 dump_pucnt.xyz

dump 3 cm xyz 10000 dumpcm_pucnt.xyz

restart 1000000 poly.restart

run 10000000

write_data final.data