Regarding system forces in NAMD

From: sruthi c k (
Date: Thu Jan 07 2016 - 06:49:51 CST

Dear NAMD developers,

I have some queries about the system forces that are calculated in the
COLVAR module implemented in NAMD.

I was trying to generate the free-energy surface by integrating the
<F_system> on the colvar obtained from an unconstrained and unbiased MD

I did 200 ns equilibrium molecular dynamics (NPT) simulation (NO BIAS
APPLIED) and collected the system force acting on the colvar. The
<F_system> for a given bin of the colvar was then calculated by taking the
average of the system forces corresponding to the rg values falling in that

What I observe is that <F_system> is a straight-line but it is all negative
numbers. I was expecting that <F_system> will be equal to zero where the
histogram of colvar has a peak and +ve and -ve on either side of it.

Could you please help me in understanding this?
Thanks in advance


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