Need help on tensile simulation of nano wire using Add / Ave Force Command...


I am simulating the tension behavior of a nano wire with one end fixed and other end pulled with Ave force or Add force command. I was expecting a net stretching due to the applied force after some time steps,however, I have noticed some peculiar behavior. When I am loading using Ave force command, the system breaks down near the loading end (i.e., the atoms in the loading group separates). And, when I am loading with add force command, the system exhibits some elongation followed by compression, i.e., net extension is zero.

Here,both the equilibrium and loading is done using NVT ensemble (as the energy is added to the system through application of a force). The boundary conditions are S S P, with non-periodic in loading direction (x direction) .The load on each atom is applied as a function of time step. I am not able to figure out were I am going wrong, I would appreciate any useful input to resolve this issue.

Following is the input script:

------------------------ INITIALIZATION ----------------------------

units metal
dimension 3
boundary s s p
atom_style atomic
variable latparam equal 3.615

----------------------- ATOM DEFINITION ----------------------------

lattice fcc {latparam} region whole block 0 4 0 20 0 4 create_box 1 whole lattice fcc {latparam} orient x 1 0 0 orient y 0 1 0 orient z 0 0 1
create_atoms 1 region whole

neighbor 2 bin
neigh_modify delay 0 every 1 check yes

------------------------ FORCE FIELDS ------------------------------

pair_style eam
pair_coeff * * Cu_u3.eam

------------------------- GROUP DEFINITION ---------------------------------

region 1 block INF 2 INF INF INF INF units lattice
group lower region 1
region 2 block 18 INF INF INF INF units lattice
group upper region 2
group boundary union lower upper
group mobile subtract all boundary
group mobupp subtract all lower

------------- EQUILIBRITION NVT----------

reset_timestep 0
timestep 0.001
velocity all create 300 12345 mom yes rot no
fix 1 all nvt temp 300 300 0.1 drag 1

#thermo 1000
#thermo_style custom step lx ly lz press pxx pyy pzz temp etotal
#dump 2 all atom 500 dump.equi.*

run 30000
unfix 1
undump 2
#- ----------- Atom count ------------------

variable ratoms equal count(upper) # Total Number of Upper Group Atoms

------------ Setting The lower Group atoms as Fixed (Freezing the Lower group atoms) -------------------

velocity lower set 0 0 0
fix 2 lower setforce 0.0 0.0 0.0

------------ Force Settings -------------

variable p equal 0.000005
variable tforce equal v_p*elapsed/100000 # Total Force on the upper group of atoms
variable aforce equal v_tforce/v_ratoms # Average Force on each atom in upper group

------------ Loading -------------------

fix 32 upper aveforce v_aforce NULL NULL # Average Force Option
#fix 32 upper addforce v_aforce 0.0 0.0 # Add Force Option

fix 14 mobupp nvt temp 300 300 0.1 drag 0.0

#thermo 100
#thermo_style custom step lx ly lz temp etotal v_aforce elapsed
#dump 12 all custom 1000 dump.all.* id x y z fx fy fz

run 100000