[gmx-users] forces in constraint

[Beniamino Sciacca]

Dear gmx-users,
I'm working with constraint forces.
I've three DNA molecules: DNA1, DNA2, DNA3.
DNA1 is the reference_group, instead the other two molecules are the 
constrained groups.
DNA has a net negative charge, so the molecules should reject 
themeselves; but the output file (pull.pdo) of the dynamic (where each 
step show the necessary force to constraint the molecules) has a lot of 
negative value of force (meaning an attraction between the molecules...).
The mean value of this force is negative in some case...
What does it mean?
How can I exctract the true force to constraint the molecules?

Thanks in advance
Beniamino Sciacca