Scaled forces, colvars and harmonic con/restraints

From: Michelle Kuttel (
Date: Mon Nov 21 2011 - 04:45:39 CST


I am trying to calculate a simple relaxed energy profile for a dihedral angle (this is a simple sanity check - I can compare the profile to one I calculated some time ago in CHARMM).
I am using colvars and harmonic restraints to do this. My colvarsConfig file looks like this:

colvarsTrajFrequency 10000

colvar {
   name Omega
   width 0.5
   dihedral {
      group1 {
         atomnumbers { 7 }
      group2 {
         atomnumbers { 5 }
      group3 {
         atomnumbers { 20 }
      group4 {
         atomnumbers { 21 }

harmonic {
name scanOmega
colvars Omega
centers -180
targetCenters 180
targetNumSteps 10000
targetNumStages 36
forceConstant 100

In my main file, I then have the line:
minimize 370000

This works OK (though the energies for the first few stages are a bit high). HOWEVER, when I change "targetNumStages 36" to "targetNumStages 72" and "minimize 370000" to "minimize 730000", suddenly I need a much higher force of 200 for it to work. If I don't increase the force, the omega angle (recorded in the .traj file) "stalls" at -145 degrees. The angle does not rotate further than this on subsequent stages.

Why would increasing the number of stages require a higher force? I thought that the width of the omega colvar would still be the same (0.5 degrees) and therefore the scaling should be the same?
I think that I do not understand the way forces are applied under harmonic restraints and I would really appreciate some insight!

Many thanks

Dr Michelle Kuttel
Department of Computer Science
University of Cape Town
Cape Town
South Africa
PH: +27 21 6505107

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