**From:** Giacomo Fiorin (*giacomo.fiorin_at_gmail.com*)

**Date:** Wed Nov 23 2011 - 14:40:34 CST

**Next message:**Ehsan Ban: "Re: Is it possible to print out the force of one specific atom?"**Previous message:**Aron Broom: "Re: Scaled forces, colvars and harmonic con/restraints"**In reply to:**Michelle Kuttel: "Re: Scaled forces, colvars and harmonic con/restraints"**Next in thread:**Michelle Kuttel: "Re: Scaled forces, colvars and harmonic con/restraints"**Reply:**Michelle Kuttel: "Re: Scaled forces, colvars and harmonic con/restraints"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

I was not aware that the implementation of the CHARMM force field in NAMD

needed validation. And in any case, I would not take the final outcome of

an iterative calculation as the test. I would directly compare all of the

atomic forces at step 0 from the same identical structure, if I was worried

that NAMD was not implementing the CHARMM ff accurately.

"targetNumSteps" does not affect the force. Rather, the force you choose

along with all the other options will affect the result. Also, because

minimization will always fall to the nearest configuration, the initial

structure matters a lot.

What structure did you start the calculation from? That is, what was the

initial value of the dihedral?

If you minimize the structures corresponding to the three minima you say,

are these structures stable?

If you compute a relaxed energy profile between two of these minima (rather

than the full -180:180 range), what is the result?

On Wed, Nov 23, 2011 at 3:03 PM, Michelle Kuttel <mkuttel_at_cs.uct.ac.za>wrote:

*> Hi all
*

*>
*

*>
*

*> On 23 Nov 2011, at 6:41 PM, Jérôme Hénin wrote:
*

*>
*

*> > Hi guys,
*

*> >
*

*> > On 23 November 2011 16:28, Giacomo Fiorin <giacomo.fiorin_at_gmail.com>
*

*> wrote:
*

*> >> Hi Michelle. What you describe seems to me that the angle gets stuck
*

*> into a
*

*> >> local minimum. This is pretty typical when you minimize. Then when
*

*> you try
*

*> >> the next window, you don't get the results you want because your angle
*

*> is
*

*> >> stuck.
*

*> >>
*

*> >> I would start all windows from the same initial structure and change the
*

*> >> center of the static restraint, rather than changing it on the fly
*

*> while you
*

*> >> minimize. That would be most appropriate for dynamics.
*

*> >
*

*> > I am not sure that will work so well. The problem is more fundamental:
*

*> > if the potential is corrugated enough that you get stuck in local
*

*> > minima, then the relaxed scan is always tricky. I run into this once,
*

*> > and one test I did was to run the scan both ways to see if I sampled
*

*> > the same conformations - and if not, which one had lower energy.
*

*> >
*

*> > You could try scanning this angle while doing MD, to give the molecule
*

*> > a chance to optimize itself beyond the nearest local minima.
*

*> >
*

*> > Once the "true" minima are identified, you might have to run the scan
*

*> > in pieces, starting from the properly minimized structures and
*

*> > sampling the barriers around going both ways. In the worst case, the
*

*> > landscape is too complex to be sampled like this, then you'd need some
*

*> > kind of MC search rather than just a torsional scan.
*

*>
*

*> Actually, this is a very simple dihedral angle in a very small molecule:
*

*> the primary alcohol in glucose. I was running it as a test, to validate
*

*> the force field in NAMD (it's a CHARMM ff) and procedure before I continue
*

*> with more complex calculations. It really is one of the simplest cases you
*

*> can get, so colvars with "harmonic" *should* work for calculating a
*

*> relaxed map. BTW, the ABF routine works beautifully on the same dihedral,
*

*> so I think that I am forced to conclude that "harmonic" does not actually
*

*> work for calculating relaxation maps. Or at least not reliably.
*

*>
*

*> >
*

*> >
*

*> >
*

*> >> On Wed, Nov 23, 2011 at 9:16 AM, Michelle Kuttel <mkuttel_at_cs.uct.ac.za>
*

*> >> wrote:
*

*> >>>
*

*> >>> The plot thickens....
*

*> >>> After much experimenting, I have finally determined that, for this
*

*> >>> particular dihedral, the optimal "harmonic" configuration is:
*

*> >>> harmonic {
*

*> >>> name scanOmega
*

*> >>> colvars Omega
*

*> >>> centers -180
*

*> >>> targetCenters 180
*

*> >>> targetNumSteps 500 #these
*

*> >>> targetNumStages 72 #are the values
*

*> >>> forceConstant 0.005 # in question
*

*> >>> }
*

*> >>> So ,it seems that my previous values were far too "high". However, I
*

*> >>> still don't understand why, if I increase targetNumStages and
*

*> >>> targetNumSteps, I need to increase the force constant as well.
*

*> >>> "targetNumSteps" should just be number of minimization steps at a
*

*> >>> particular force, surely, and not affect the applied force, right?
*

*> But it
*

*> >>> does!
*

*> >
*

*> > Note that the force constant is in unit of kcal/mol/square colvar
*

*> > unit, where the effective colvar unit is given by the width parameter.
*

*> > Here, it is 0.5 degrees, which is very small, meaning that a force
*

*> > constant of 100 is really huge (deviating from the restraint center by
*

*> > 0.5 degrees costs 100 kcal/mol of energy). A looser restraint leaves
*

*> > more leeway for various degrees of freedom to relax, possibly yielding
*

*> > a better optimization.
*

*> >
*

*> > The reason targetNumStages matters is, you may not get trapped in the
*

*> > same local minima depending on the size of the "hops" in the reference
*

*> > torsional angle.
*

*>
*

*> But this is a simple dihedral, with only three minima (at -60, 60 and 180
*

*> degrees) - and these minima are not where it gets stuck....
*

*> I am still not clear on why "targetNumSteps" affects the force....?
*

*>
*

*> Many thanks
*

*> Michelle
*

*>
*

*> >
*

*> > Best,
*

*> > Jerome
*

*> >
*

*> >>> The use of "harmonic" to create relaxed energy maps is not documented
*

*> in
*

*> >>> the NAMD User Guide, other than the comment:
*

*> >>> "The harmonic biasing method may be used to enforce fixed or moving
*

*> >>> restraints, including variants of Steered and Targeted MD. Within
*

*> energy
*

*> >>> minimization runs, it allows for restrained minimization, e.g. to
*

*> calculate
*

*> >>> relaxed potential energy surfaces.".
*

*> >>> So, I would really appreciate it if someone can explain this to me.
*

*> >>> Pretty please?
*

*> >>> Michelle
*

*> >>>
*

*> >>> On 21 Nov 2011, at 12:45 PM, Michelle Kuttel wrote:
*

*> >>>
*

*> >>> Hello
*

*> >>> 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
*

*> >>> Michelle
*

*> >>> ---------------------------------------------
*

*> >>> Dr Michelle Kuttel
*

*> >>> Department of Computer Science
*

*> >>> University of Cape Town
*

*> >>> Cape Town
*

*> >>> South Africa
*

*> >>> mkuttel_at_cs.uct.ac.za
*

*> >>> PH: +27 21 6505107
*

*> >>> ---------------------------------------------
*

*> >>>
*

*> >>>
*

*> >>>
*

*> >>
*

*> >>
*

*> >
*

*>
*

*>
*

**Next message:**Ehsan Ban: "Re: Is it possible to print out the force of one specific atom?"**Previous message:**Aron Broom: "Re: Scaled forces, colvars and harmonic con/restraints"**In reply to:**Michelle Kuttel: "Re: Scaled forces, colvars and harmonic con/restraints"**Next in thread:**Michelle Kuttel: "Re: Scaled forces, colvars and harmonic con/restraints"**Reply:**Michelle Kuttel: "Re: Scaled forces, colvars and harmonic con/restraints"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

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