I’m modelling the pull-out test of CNT from copper matrix. I want to obtain the force-displacement curve.
I applied (fix move) to the carbon atoms as follows;
fix 9 C move linear 0 0 0.5
the step time is 0.0005 ps
The problem is that when i get the total force of Carbon atoms by fcm(), the force is fluctuating between negative and positive values while the displacement of carbon atoms is always increasing. What i know is that while the displacement increases, the force should be always positive.
Can I get help considering that??
I need to know the problem.
My best regards
First, I suggest you check the CNT moving rate of 0.5 (default lattice units). It may not be what you think if the intension was 0.5 A/ps. (if metal units).
Secondly, “What i know is that while the displacement increases, the force should be always positive” is not necessarily true. For an example, please see “http://www.sciencedirect.com/science/article/pii/S0927025611006756”
Ray
I'm modelling the pull-out test of CNT from copper matrix. I want to obtain
the force-displacement curve.
I applied (fix move) to the carbon atoms as follows;
fix 9 C move linear 0 0 0.5
the step time is 0.0005 ps
The problem is that when i get the total force of Carbon atoms by fcm(), the
force is fluctuating between negative and positive values while the
displacement of carbon atoms is always increasing.
What i know is that while the displacement increases, the force should be always positive.
where do you know this from? my expectation would be different and
exactly as your observation is. i have even been able to *feel* these
things through using force feedback interactive simulations:
https://sites.google.com/site/akohlmey/software/vrpn-icms#TOC-Demos
Can I get help considering that??
I need to know the problem.
my guess is that your simulation is not a correct model of what you
expect it to be.
axel.
I appreciate your responses.
To Mr. Shan, yes I mean 0.5 A/ps and I added (units box) to the (fix move), knowing that I’m using (metal) units. The weird thing is the velocity is fluctuating from negative to positive values and the displacement is still increasing.
To Dr. Axel, yes may be the problem with some mistakes I’ve done in the model as you said, so here is some of the commands after equilibration;
fix 9 C move linear 0 0 0.5 units box
fix 1 others nvt temp 1 1 0.005
compute 4 upper reduce max c_3[3]
variable dz equal c_4
variable fz1 equal fcm(C,z)
thermo_style custom step temp v_fz1 v_dz
run 200000
#C: Carbon atoms
#others: Carbon atoms + Copper atoms - some fixed copper atoms
#upper: the CNT upper ring atoms
Thank you
I appreciate your responses.
To Mr. Shan, yes I mean 0.5 A/ps and I added (units box) to the (fix move), knowing that I’m using (metal) units. The weird thing is the velocity is fluctuating from negative to positive values and the displacement is still increasing.
What we have been trying to convey is that this is not weird. The model does what it is told to, and just because the result does not meet your expectation, it should not be called weird. Rather, I suggest you check your model, structure, set up, rates, potential, etc.
Commands below are not complete, so it is still difficult to know what went wrong. My suggestions is to read more papers and perhaps try to reproduce results from one that is do-able with LAMMPS.
Ray