# Re: ForceConstant in Umbrella Sampling

From: Chris Harrison (charris5_at_gmail.com)
Date: Fri Oct 28 2011 - 18:10:40 CDT

For what you're asking: K=25, not K/2. But in case you use a
multidimensional colvar, I think you should be aware
of how the force is distributed in multidimensional restraints:

- forceConstant < (harmonic) Scaled force constant (kcal/mol) >
Acceptable Values: positive decimal
Default Value: 1.0
Description: This defines a scaled force constant for the harmonic
potential. To ensure consistency for multidimensional restraints, it is
multiplied internally by the square of the specific width for each
colvar involved (which is 1 by default), so that all colvars are
effectively dimensionless and of commensurate size. For instance,
setting a scaled force constant of 10 kcal/mol acting on two colvars, an
angle with a width of 5 degrees and a distance with a width of 0.5 Ang
will apply actual force constants of 0.4 kcal/mol/degree for the angle
and 40 kcal/mol/Ang for the distance.

Best,
Chris

```--
Chris Harrison, Ph.D.
Theoretical and Computational Biophysics Group
NIH Resource for Macromolecular Modeling and Bioinformatics
Beckman Institute for Advanced Science and Technology
University of Illinois, 405 N. Mathews Ave., Urbana, IL 61801
char_at_ks.uiuc.edu                          Voice: 773-570-0329
http://www.ks.uiuc.edu/~char              Fax:   217-244-6078
> Date: Fri, 28 Oct 2011 17:38:24 +0000
> To: "namd-l_at_ks.uiuc.edu" <namd-l_at_ks.uiuc.edu>
> Subject: namd-l: ForceConstant in Umbrella Sampling
>
> Dear NAMD Users,
>
>
> I am preforming umbrella sampling using collective variable module as distanceZ of NAMD.2.7 version. I am confused about the input value of “forceConstant”, whether is it K or K/2 kcal mol^-1 A^-2. Suppose I want to input forceConstant as 25 kcal mol^-1 A^-2. So in the input file do I need to input 25 or 12.5. Could you please give me reply, it will be great help for me.
>
>
> Have a nice day,
>
>
> Thanks,
>
> Maiti
```

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