[gmx-users] constraint force

[Kay Gottschalk]

>
Hi there,
I have (again..) some questions about the constraint force.
a) What are the units? I couldn't really find it in the manual.
b) The manual states that a negative constraint force means attraction, 
while a positive repulsion. Having in mind that there is an approach to 
calculate the free energy profile by integrating the average constraint 
force, this appears to be a problem. If you integrate the constraint 
force as given by gromcas, you get a positive free energy when two 
attracting particles approach each other. Am I getting this wrong?
c) How important is the choice of atoms I use to constrain the 
distance? Does it make a difference to take just the CA as opposed to 
the whole protein?

Since there is some discussion in the literature about when (if) it is 
appropriate at all to get a free energy profile by integrating the 
average constraint force, I would be grateful for some opinions on this 
matter.

Thanks a lot!
Kay.