Re: forces saved in forcedcd

From: Xiaohu Li (
Date: Thu May 03 2012 - 22:40:34 CDT

I first tried to use the colvar option to apply a harmonic constraint between the center of mass of the liquid slab and the ion, however, since my slab has tens of thousands of atoms, the un-parallelized colvar code slow down my
simulation by a factor of ten!
So instead, I applied a very large harmonic constraint on the COM of the slab using the SMD option and another harmonic constraint on the ion using regular harmonic constraint.
Of course, the slab has been equilibrated.
Another thing is that when I inserted my ion into the slab, I used VMD to deleted A solvent molecule nearby to avoid any close contact. I'm guessing this effect is small since I have like 700 solvent molecules.
I ran each window dynamics for 600ps, where the first 100ps was discarded as equilibration.


On May 3, 2012, at 10:05 PM, JC Gumbart wrote:

It's easier to set up, but should be equivalent in the end.

How did you prepare and run your US sims? I cannot for the life of me figure out why you are getting an oscillatory pattern from US.

On May 3, 2012, at 9:57 PM, Xiaohu Li wrote:

I have used umbrella sampling by either using WHAM or averaging the forces the the window.
How's ABF compared to those approaches? Are there any qualitative differences?


On May 3, 2012, at 8:11 PM, JC Gumbart wrote:

How are you calculating the PMF? Did you try, say, ABF?

On May 3, 2012, at 7:51 PM, Xiaohu Li wrote:

Hi, Chris,
      Thanks for taking time for pondering on my question.
      In my case, I would have to stick with the NVT simulation since I'm using the drude polarizable force field. The reason I ask is because I'm doing a PMF of ions pulled out of bulk(glycerol), which the reaction coordinates is quite simple as the surface normal direction. I'm seeing something quite difficult to explain since from a very naive understanding, the PMF should be flat deep in the bulk(as seen in water, methanol, etc...). I'm getting quite oscillatory pattern in the bulk and the i'm talking about several kcal/mol oscillation(using either wham or the direct forces). That leads me to this question. Something I notice is that the frequency of the Langevin friction affect the self-diffusion. So extrapolate from that, it should affect the forces.
      Any more opinions on that?


On May 3, 2012, at 6:42 PM, Chris Harrison wrote:

Yes. However, in the limit of converged sampling the Langevin forces are theoretically canceling out to noise. ...theoretically. In practice, it is entirely possible that when using a Langevin thermostat you might see the periodicity produced from the kicks when analyzing the forces.

"Will the Langevin forces affect PMF calculations?" That entirely depends on the PMF you are performing. Many considerations go into answering that question. Consider: Are the forces being predominately measured in the PMF of a small enough scale that the Langevin kicks can be resolved? If performed correctly, the Langevin forces should look like noise. Another consideration: a purist would argue to perform an NVE simulation, removing as many external forces/perturbations from the system as possible.

Unfortunately the answer to your question is: yes, they can affect PMFs, but a) will it matter for the question you are asking, and b) you will have to test to actually determine for sure; or alternatively pursue an NVE ensemble simulation.


On Thu, May 3, 2012 at 3:43 PM, Xiaohu Li <<>> wrote:
Dear NAMD users,
        I have a question regarding the forces saved in forcedcd file. suppose I'm doing NVT using Langevin dynamics, will the forces saved in forcedcd contain the random forces from Langevin dynamics?
        Do those random forces affect, say, PMF calculations if they are included?

Chris Harrison, Ph.D.
NIH Center for Macromolecular Modeling and Bioinformatics
Theoretical and Computational Biophysics Group
Beckman Institute for Advanced Science and Technology
University of Illinois, 405 N. Mathews Ave., Urbana, IL 61801       Voice: 773-570-6078                          Fax: 217-244-6078

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