[gmx-users] pull=constraint gives zero forces

[Christopher Neale]

How can there be forces for holonomic constraints? Is this described by an equation in the manual?
Just because there are values in the pullf.xvg file does not mean that these values are forces.
If they are forces, what is the force constant and what is the equation that defines this force?

Thank you,
Chris.

-- original message --

But for GMX 4.0.7 there are forces in the pullf.xvg. The forces which 
arise rom the contraint the hold the two groups fixed. I use them for 
thermodynamic integration...

I use the following mdp-parameters, probably this gives you an idea what 
you might make different:
; AFM OPTIONS
pull                =  constraint
pull_geometry       =  distance
pull_dim            =  Y Y Y
pull_start          =  yes
pull_nstfout        =  10
pull_nstxout        =  500000
pull_ngroups        =  1
pull_group0         =  REF
pull_group1         =  ZUG
pull_rate1          = 0.000
pull_init1          = 0.000
pull_constr_tol          = 1e-06

But as Chris said, better post the complete mdp-file (probably for both 
GMX 4.0.7 and 4.5.x) so we can comment on them.

Grettings
Thomas