I’m trying to understand how the “fix smd” functions in terms of the force and velocity direction.
I tried to test on a very simple case which is basically a sphere carved out of a regular fcc crystal lattice. I also set the cut off so small ( = 0.1) so the atoms of the sphere have no interaction. I try to pull or push this sphere with “fix smd” to understand its behaviour. I have attached the input files and the log files for two cases:
a. Tether point is below the centre of mass of the sphere and cvel is positive.
b. Tether point is below the center of mass of the sphere and cvel is negative.
For case a the sphere moves upward (in the direction of the velocity) uniformly which make sense but:
— Why in case b while the spring is moving down the sphere just goes down to up to the initial tether point of the spring and just oscillates around that point while the spring is still moving downward? Shouldn’t it show a behaviour as case a except that its going down?
Note: For spring constant I chose a large value to make very strong; and for cvel I chose a value to move sphere in few timesteps. Dt = 0.005 (default value) in the both cases.
I’ve also browsed most of the posts on “fix smd” and saw a similar discussion and a reference to this part of the documentation there:
"In cvel mode the distance R is incremented or decremented monotonously according to the pulling (or pushing) velocity."
but I couldn’t figure out how to justify the behaviour of case a and b.
I appreciate your time a help in advance.
in.a (815 Bytes)
in.b (816 Bytes)
log.b (113 KB)
log.a (113 KB)