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