Force constant for Umbrella sampling and WHAM

From: Udaya Dahal (
Date: Wed Feb 13 2019 - 12:07:42 CST


I am using colvar module for running umbrella sampling (1D) with window
width of 1A and a force constant (1 kCal/mol). Since, NAMD defines energy
as (k x^2) to rescale the force constant (as can be seen in log file by
modifying the width of the window) but in case of WHAM ( by Grossfield) ,
the energy is defined as 1/2 k x^2. I am wondering if I need to use
different force-constant to match the exact value used in simulation -- in
my case 2kCal/mol/A^2 while doing the free energy calculation.

I would like to point out that I am getting exactly double the value of the
free energy when I doubled the force constant.

I appreciate your help.

Best Regards,
Udaya Dahal

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