I am using umbrella sampling technique to constraint the distance between two segments of protein defined by the reaction co-ordinate. As per the theory, the distance constraint with a proper force constant should ideally sample around the given distance. For example, let's say, for 9 A distance constraint, the segments can interact between 8 and 10 A, (9 + 1) However, for me, it samples between 10 and 11. Sampling of (9 + 1) happens with a distance constraint of 8 A.
If there is a barrier at these distances, a distance constraint of 8 A should not sample around 9. Why is the shift happening. I have tried using higher force constants. But it doesnt work. Can somebody explain why this happens?
I have attached the histogram to explain my problem better. Please advise.