About fix SMD

Dear All,

I want to perform SMD simulation. Some issues confused me about that. I use cvel type and tether mode. Let me consider the following example,

fix pull cterm smd cvel 20.0 -0.0001 tether NULL NULL 25.0 0.0

In the above example, let me again to assume group cterm initially placed at z=0. In this case, we have a spring (with spring constant 20.0) between tether point and group cterm. The tether point is fixed and does not move. The distance between center-of-mass group cterm and tether point is 25A at t=0 and equals the length of spring ®. Also, the length of spring ® is decremented monotonously according to the velocity -0.0001.

Now my questions are:

1- Does the left head of spring (and also group cterm) move toward to the tether point?
2- Does the applied force to the group cterm is equal F=-KR? In this case, for so far tether point from z=0, we will apply much larger force and therefore the results for PMF will be different. I mean that the PMF will depend on the R?! or even on K?!
3- Does the spring shortened with the velocity of -0.0001 or compressed?

Please clarify me about this fix,
Thanks in advance