Constant force steered molecular dynamics (CF-SMD)

GROMACS version: 2018.1
GROMACS modification: No
Here post your question : how to perform steered molecular dynamics with application of constant force (CF-SMD)?
From the mdp options of COM pulling from gromacs manual, I tried to perform CF-SMD (mdp options and result of this simulations is attached below), I got constant force graph but molecule did not extend (extension=0).
; Pull code
pull = yes
pull_ncoords = 1 ; only one reaction coordinate
pull_ngroups = 2 ; two groups defining one reaction coordinate
pull_group1_name = Chain_A
pull_group2_name = Chain_B
pull_coord1_type = constant-force ; harmonic potential
pull_coord1_geometry = distance ; simple distance increase
pull_coord1_dim = Y N N
pull_coord1_groups = 1 2
pull_coord1_start = yes ; define initial COM distance > 0
pull_coord1_k = 500 ; kJ mol^-1 nm^-2

Can anyone suggest me if I am missing something or this is not the way for performing constant force SMD and if so how I can perform CF-SMD.