Little strain difference in tensile test

LAMMPS LAMMPS Beginners
1

Hello LAMMPS users, I am using windows 30 july 2021. units are real. I am performing a tensile test on a single graphene, first row of graphene (named as pullatoms) is pulled at specified velocity in the x-direction, and last row of graphene (named as stationary atoms) is remained as fixed. The velocity is mentioned, as it must give strain of 0.4 in the x-direction, but the strain in the x-direction is obtained as 0.372. Am I missing something below.

##-------------------Tensile test begin------------------------------------##
velocity stationaryatoms set 0.0 NULL NULL

fix 3 stationaryatoms setforce 0.0 NULL NULL

velocity pullatoms set ${velocity_x} NULL NULL

fix 5 pullatoms setforce 0.0 NULL NULL

fix 1 all npt temp 300 300 100 y 0 0 1000

fix 4 all ave/time 1 100 100 c_forceall[1] c_forcepullatoms[1] c_forcestationaryatoms[1] forcemobile[1] c_sigmaall[1] c_sigmastationaryatoms[1] c_sigmapullatoms[1] c_sigmamobile[1] v_Lx v_Ly v_Lz v_Vol file D:\1_graphene_T15\tensile\ave_time\average_ave_time.txt

Thanks and regards,
Ankit

2

If you define a lattice, then you may need to use the units box keyword for the velocity command.

3

##-------------------Tensile test begin------------------------------------##
velocity stationaryatoms set 0.0 NULL NULL units box

fix 3 stationaryatoms setforce 0.0 NULL NULL

velocity pullatoms set ${velocity_x} NULL NULL units box

fix 5 pullatoms setforce 0.0 NULL NULL
,
Hello Michael, I used units box in velocity command but still getting same strain 0.372 not getting 0.4