Hello, all

Recently, I did a simple simulation, which was a tension

of a bar by fixing one end and exerting force on boundary

atoms of the other end (called hi_fixed hereafter).

There were three stages in my simulation:

first, relax the sytem to equilibrium,

then, exert tension gradually from 0 to the maximum,

finally, keep the maximum force for some time steps.

The corresponding statements I used in the last two stages are as follow:

dimension 3

boundary p p s

variable runstep_loading equal 100000

variable runstep_relax equal 40000

variable myscalor equal 1e0

variable force_every_atom equal 0.0028871*\{myscalor\}
variable force\_every\_atom\_loading equal &
{force_every_atom}*elaplong/${runstep_loading}

# prepare boundary

# -----------------------

velocity lo-fixed set 0.0 0.0 0.0

fix 1 lo-fixed setforce 0.0 0.0 0.0

velocity hi-fixed set 0.0 0.0 0.0

# loading

# ------------

fix 4 hi-fixed aveforce 0.0 0.0 v_force_every_atom_loading

fix 5 all nve

run ${runstep_loading}

unfix 4

unfix 5

# relax

# ------------

fix 4 hi-fixed aveforce 0.0 0.0 \{force\_every\_atom\}
fix 5 all nve
run {runstep_relax}

unfix 4

unfix 5

The center-of-mass of the boundary atoms group hi_fixed

is also computed :

compute hi_com hi-fixed com

However when I did post-processing, I found the center-of-mass

of hi_fixed at gradual loading stage oscillated symmetrically

around the center-of-mass of hi_fixed when equilibrium is obtained

at the first relax stage. But when it entered the stage of keeping

the maximum load (the third stage), the oscillation center of the

center-of-mass of hi_fixed showed a jump and was discontinuous with

the second stage distinctly.

I also outputed the total force on the group of hi_fixed before

fix aveforce took action. It showed a mean value of 0 at the gradual

loading stage and sudden jump when entered the third stage.

I think it is very strange. Could anyone point out the problem?

Thank you in advance!

Wang Kai