Change sign of force on constraints and pair force
The change of sign makes the value of the force consistent with the definition of the force ( -grad(E) ) A positive value of the constraint force tends to increase the constraint value. The change of sign of PairForce makes it such that a positive PairForce value tends to increase the bond length. The value of the PairForce needed to compensate for a distance constraint is opposite to the force acting on the constraint.
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