Re: rigidBonds water and langevinHydrogen off (no problem!)

From: Aron Broom (broomsday_at_gmail.com)
Date: Thu Mar 01 2012 - 14:32:20 CST

Hi Victor,

I did take the mass of just the oxygen. I thought that was the correct
thing to do, no? Maybe I'm completely off base with that. If I take the
mass of the whole water then the temperature of the water is 330 K. This
does mean the water and protein are ~the same temp (although that is now
~30 K higher than where the thermostat was set), but is this right? I
understand that the water is effectively moving as an 18 amu rigid unit,
but it's not clear to me that it should be counted in that way for kinetic
energy, because those 2 amu hydrogens don't have all their degrees of
freedom.

Honestly I'm out of my league on that question.

~Aron

On Thu, Mar 1, 2012 at 2:56 PM, Victor Ovchinnikov
<ovchinnv_at_georgetown.edu>wrote:

> Aron,
> Just a random thought. Could it be you took the mass of oxygen rather
> than that of the whole water molecule? 16/18 * 300K ~ 266K ?
> Best,
> Victor
>
>
> On Thu, 2012-03-01 at 14:27 -0500, Aron Broom wrote:
> > Dear JC,
> >
> > Thanks for the reply!
> >
> > Actually, I did only count the oxygens when calculating the KE for the
> > water, this is what I meant by "determining the temperature of the
> > non-rigid parts of the system" in my original message, but I
> > understand that it's not worded in an extremely clear manner. Same
> > thing when looking at the rest of the system whenever I was using
> > SHAKE, I ignored all rigid hydrogens. But you are absolutely correct
> > that counting those without accounting for the reduced degrees of
> > freedom would be bad. So that part isn't an issue.
> >
> > Also, I didn't derive a conversion factor from nothing, I converted
> > the speeds which are in angstroms/ps to m/s, and the mass from amu to
> > kg and then used a boltzmann constant of 1.380e-23 J/K in the fitting,
> > so there was nothing fishy about that, I wasn't just making it work.
> > Perhaps the units have not changed, I only suggested that for others
> > because in the tutorial provided the values you would get from
> > following it line-by-line don't work with the boltzmann constant
> > mentioned in the tutorial.
> >
> > In terms of the statistical relevance, I completely agree that my
> > 30,000 water oxygens are going to make a good fit to the boltzmann
> > distribution, whereas 1000-2000 (depending on SHAKE) protein atoms are
> > not going to be as great, and looking at the fits confirmed this. I
> > did, however measure those temperatures at 6 separate time points
> > across 60 ns of simulation, and the standard deviation on the protein
> > was only +/- 5 K, so its being ~20K hotter is statistically meaningful
> > (the standard error in this case is less than 5K). I should have put
> > the actual error in my original message to make this more clear.
> >
> > In your test how large was the system, that is, how many water
> > molecules and how large of a solute? Also, how long did you simulate
> > for? I failed to mention in my original message that the system had
> > been pre-equilibrated with for 20 ns before the 60 ns run, so I
> > imagine it might take some time to see these effects creep up (that
> > is, the first timepoint that I measured at was 30 ns into the run
> > effectively), and it might be dependent partially on the size of the
> > water box. Also, I was using a Langevin-Nose barostat, although I
> > suspect that wouldn't be a contributing factor, it's worth pointing
> > out.
> >
> > I agree completely that one shouldn't state thing prematurely. I feel
> > that I did a fair amount of testing in this case, and confirmed that
> > everything I was doing was sensible (and certainly never derived
> > random things to make the answer fit), but it could easily still be
> > the case that I've missed something critical, or I had some other
> > setting incorrectly that caused this. And as I only did 4 ns with the
> > different settings to confirm a restoration of the proper
> > distribution, that part is not strictly speaking unassailable
> > statistically.
> >
> > ~Aron
> >
> > On Thu, Mar 1, 2012 at 1:49 PM, JC Gumbart <gumbart_at_ks.uiuc.edu>
> > wrote:
> > Dear Aron,
> >
> >
> > The statistical mechanics of rigid waters is different from
> > that of flexible waters. The energy in the Maxwell-Boltzmann
> > distribution for a given water molecule is no longer
> > sum(1/2*mi*vi^2) for all three atoms, but instead
> > 1/2*m_total*v_oxygen^2 (similar for other rigid hydrogens and
> > their parent atoms). The velocities reported for the
> > hydrogens by NAMD, I believe, are meaningless. When you use
> > this formulation, you recover the proper distribution and
> > temperature for the system and its components. I did find a
> > slight difference in the temperature of the solute and water
> > (the solute was about 5% lower), but I don't believe it is
> > significant. The water has a lot more particles, and
> > therefore generates more converged statistical averages (the
> > tutorial even states that agreement only within +/- 10 K is
> > expected).
> >
> >
> > Also note that the units have not changed in the velocity
> > output by NAMD at any time. I suspect you only obtained the
> > proper temperature for the full system because you empirically
> > derived a conversion factor that made the data fit what you
> > expected.
> >
> >
> > You might also look at the code that NAMD itself uses to
> > calculate temperature, based on the equipartition theorem,
> > noting in particular that it removes constrained degrees of
> > freedom.
> >
> >
> > It is certainly worthwhile to verify for yourself the
> > underpinnings of an MD code, but be careful about stating
> > there is something wrong prematurely!
> >
> >
> > JC
> >
> >
> > On Feb 28, 2012, at 9:58 PM, Aron Broom wrote:
> >
> > > Dear NAMD users,
> > >
> > > I've looked around on the mailing list and couldn't find a
> > > clear answer to as to whether or not it is supposed to be ok
> > > to use rigidBonds water (say with TIP3P) but then leave the
> > > solute (say a protein) flexible with a 1 fs timestep and
> > > langevinHydrogen off in order to save on calculating
> > > collisions with the large number of rigid hydrogens in the
> > > sample. It might seem like one should not use
> > > langevinHydrogen off as long as there are any flexible
> > > hydrogens, but several sources, including an NAMD tutorial
> > > (
> http://www.ks.uiuc.edu/Training/Tutorials/science/channel/channel-tutorial-files/ABF/win1/win1A.conf)
> have exactly this setup.
> > >
> > > Well, I believe I have confirmed that you should NOT do
> > > this. In looking at a 60ns simulation of ~2000 protein
> > > atoms in an ~100,000 water atom box, and determining the
> > > temperature of the non-rigid parts of the system by
> > > calculating their kinetic energies (KE=0.5mv^2), binning
> > > them, and then fitting to the boltzmann distribution ( y =
> > > (2/sqrt(Pi * (KbT)^3 )) * sqrt(x) * exp(-x/KbT) ), I find
> > > that the temperature of the whole system is fine (set to 298
> > > K, and comes out on average at 297 K), but the temperature
> > > of the protein alone is ~20-25 K higher than the surrounding
> > > solvent throughout the simulation.
> > >
> > > This is a big problem, especially for studying ligand
> > > binding, and I imagine for anything having a 20-25 K
> > > difference across the solvent-solute interface is bad, not
> > > to mention just being incorrect.
> > >
> > > I tested running an extra 4ns from the end of my 60 ns, with
> > > two changes in NAMD parameters:
> > >
> > > 1) just turn langevinHydrogen on, which comes with a 5%
> > > performance penalty, and
> > >
> > > 2) leave langevinHydrogen off, but change rigidBonds water
> > > to rigidBonds all, and change from a 1 fs timestep to a 2 fs
> > > timestep, in which you gain ~30% performance, but some
> > > people are sceptical of the results.
> > >
> > > Both of these changes alone are capable of bringing the
> > > system back to a reasonable configuration in which the
> > > protein and solvent have approximately the same temperatures
> > > (the protein was still slightly hotter (~5 K, but perhaps a
> > > gamma of 1/ps is not fast enough to see complete equilibrium
> > > in 4ns).
> > >
> > > Has anyone else seen this and can confirm that the setup of
> > > using ridigBonds water and langevinHydrogen off are not to
> > > be used together when you have a solute that has hydrogens,
> > > despite this being somewhat common practice? If anyone
> > > wants to test the temperatures of various parts of their
> > > systems from existing *.vel files, I just followed the
> > > instructions on this page
> > > (
> http://www.ks.uiuc.edu/Training/Tutorials/namd/namd-tutorial-unix-html/node13.html),
> but note that the units in the .vel files must changed since this was
> written, so you'll have to convert amu to kg, and angstrom/ps to m/s, and
> then use the correct boltzmann constant for those units.
> > >
> > > ~Aron
> > >
> > >
> > > --
> > > Aron Broom M.Sc
> > > PhD Student
> > > Department of Chemistry
> > > University of Waterloo
> > >
> >
> >
> >
> >
> >
> > --
> > Aron Broom M.Sc
> > PhD Student
> > Department of Chemistry
> > University of Waterloo
> >
>
>
>

-- 
Aron Broom M.Sc
PhD Student
Department of Chemistry
University of Waterloo

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