Re: Umbrella sampling-NAMD

From: Aron Broom (
Date: Thu Dec 15 2011 - 11:11:04 CST


If you had a very simple system like two molecules in the gas phase then I
would be worried by the disagreement between Umbrella Sampling and ABF, but
given the complexity of your system, are you sure that the ABF calculations
have given an accurate PMF? Have you checked that doubling the sampling
parameter and/or doubling the simulation time has no effect on your ABF
PMF? I would intuitively suspect that the difference is because one of
your analysis methods has not really converged properly.


On Thu, Dec 15, 2011 at 10:24 AM, Parisa Akhski <> wrote:

> **
> Dear NAMD experts,
> I am using NAMD to run Umbrella Sampling on an ion channel. I am using a
> "harmonic force constant" to keep the ion in the center of bin in each
> window and I know that this value is scaled by the square of the specific
> width as mentioned in the manual. So, if I define "forceConstant=1" and use
> width=0.1, NAMD actually applied K=100 in the calculations.
> Then, I am processing the trajectory files by using WHAM to produce PMF. I
> know that WHAM assumes that the biasing potential is V=1/2K(X-X0)^2 whereas
> CHARMM just defines it as V=K(X-X0)^2. This means if I am using K=100 in
> NAMD, I should define K=200 in WHAM.
> Since I already have the PMF profile from ABF calculations for the same
> system, I know how the profile should look like and what are the energy
> barriers in my system. Comparing theses results with the ones from Umbrella
> Sampling and WHAM, the shape of the profile makes sense but not the amount
> of the barriers. However, if I change the K value from 200 in WHAM to 50,
> everything makes sense. I double checked this with WHAM experts to make
> sure I am using the correct K (200) in WHAM.
> Could you please let me know whether you have any suggestions,... to solve
> this issue?
> Many thanks for your time,
> Parisa

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