From: Peter Freddolino (petefred_at_ks.uiuc.edu)
Date: Mon Nov 24 2008 - 21:29:34 CST
I'm a little unclear on what you're doing... are you labeling as a
"state" each single frame from your trajectory? This is unlikely to give
good statistics as you need to average over solvent conformations and
such (not to mention local fluctuations in the protein). Or are you
defining some set of reduced coordinates to calculate your free energy
surface? In the latter case I'd recommend looking over some papers in
the literature with complete solutions for applying WHAM-style analysis
to replica exchange simulations:
J. Phys. Chem. B, 2005, 109 (14), pp 6722–6731
JCTC, 2007, 3, pp 26-41
All you should need to do to run at constant volume is turn off
langevinPiston and comment out the settings following it.
Steven Samuel Plotkin wrote:
> Hi Peter,
> I think you are right, and this was why I took the total energies
> initially, which includes both KE and PE of the whole peptide plus
> water. This is what governs the original sampling probabilities, so I
> have discarded the idea of cherry-picking parts of the system. To find
> the re-weighted probabilities for each state I have been simply
> reweighting each state by the factor:
> exp( E_i ( beta_i - beta_targ) )
> where E_i is the total energy of the state, beta_i is 1 over kB times
> the temperature at that sample time, and beta_targ is 1 over kB times
> the target temperature. This should be fine I think, at least for
> const volume simulations.
> After doing this re-weighting however, the new exponents in the
> Boltzmann factors are (when sorted from highest)
> so one particular state ends up having all the Boltzmann weight in the
> partition function (i.e. exp(16.3) more weight than the next
> weightiest state). I didn't think this was reasonable, it says entropy
> isnt important in the partition function. What are the solutions for
> this problem? Perhaps more sampled configurations, but I have 8000 now
> which I thought should be enough to at least get a rough idea what the
> potential looks like. Maybe I need a narrower temperature range- the
> beta_i - beta_targ factors are making the exponents too large. However
> previous studies of REMD seem to have gone to 500K without
> difficulty... I must be missing something.
> I did not add a pressure correction to the replica exchange scripts. I
> hadn't thought this through carefully enough and didn't know the REMD
> scripts had not accounted for it. According to the reference you
> mentioned, there is an extra term in the transition matrix, and an
> extra term in the probabilities ~ Vi^N exp(-beta P V_i ). I will have
> to check how large this factor is. I don't think the volume
> fluctuations should be very large but I could be wrong. Then I would
> have to rerun my simulations at constant volume. I assume from reading
> some previous posts on the forum that removing all statements
> pertaining to pressure and variable volume such as
> useGroupPressure yes ;# needed for rigidBonds
> useFlexibleCell no
> useConstantArea no
> langevinPiston on
> langevinPistonTarget 1.01325 ;# in bar -> 1 atm
> langevinPistonPeriod 100.
> langevinPistonDecay 50.
> from the conf file should do the trick.
> Thanks for any help,
> Peter Freddolino wrote:
>> By the way, regarding the text below... I do not think that you
>> should be taking the energies of a subset of your system to use in
>> the analysis. You really ought to be using the total energy of your
>> system for doing the type of analysis that you're doing; otherwise
>> you're basically cherry-picking part of the total energy of your
>> system without regard for the system as a whole. There are several
>> variants of WHAM for analyzing replica exchange data in the
>> literature that should be applicable to your problem. Also, the namd
>> replica exchange scripts don't contain a pressure correction (Chem.
>> Phys. Lett. 335:435–439.), so you should be running each simulation
>> at constant volume unless you've added it...
>> Steven Samuel Plotkin wrote:
>>> Dear NAMD users,
>>> I have run namdEnergy on a dcd file, printing out All energies.
>>> First I noticed the kinetic energy was not printed, then I compared
>>> the "TOTAL" potential energy from namdEnergy with the TOTAL energy
>>> minus the KINETIC energy in my log file. The two are quite different
>>> -- about 370 kcal/mol on average. I'm confused by this, I'm not sure
>>> what is being calculated differently? I've provided links to my dcd,
>>> log, namdEnergy settings, and conf file are below. I've also posted
>>> a long version of my question below to help put it in context.
>>> (I'm using namd 2.6 and vmd 1.8.6).
>>> Thanks for any help,
>>> Steve P.
>>> log and dcd file:
>>> Conf files:
>>> Screenshot of my namdEnergy gui, with settings:
>>> xsc file (if needed):
>>> /Long version of my question:/
>>> I have been able to modify the Alanin replica exchange tcl code to
>>> run on a small peptide which has been solvated and is in ionic
>>> solution. I am using namd 2.6 on an x86_64 amd cluster. I ran 16
>>> replicas on 8 CPUs across 2 nodes of a cluster, with temperatures
>>> ranging from 280K to 440K. My conf file has some new information not
>>> in the alanin example because of my solvated box: I have
>>> cellBasisVectors, wrapWater, wrapAll, PME, and Pressure control
>>> options including langevinPiston. I was careful to choose the
>>> langevinPistonTemp to be $NEWTEMP in replica_exchange.tcl.
>>> I want to construct a free energy profile for the small peptide.
>>> However this must be obtained at one (target) temperature, so I
>>> reweighted all states by what their Boltzmann weights should have
>>> been at the target temperature. This doesn't work for me: because
>>> the system includes the peptide plus the whole water box, after
>>> re-weighting, one particular state ends up having all the Boltzmann
>>> weight in the partition function. I didn't think this was feasible.
>>> So then what I must use is the total energy (KE +PE) of the protein
>>> itself, which includes self interactions /plus/ interactions with
>>> water and ions. Then I can reweight these energies at the target
>>> temperature and get a new partition function and free energy
>>> profile. I thought namdEnergy would be the tool for this.
>>> As a sanity check, I ran namdEnergy on one of my dcd files, printing
>>> out 'All' energies. First I noticed the kinetic energy was not
>>> printed, then I compared the "TOTAL" potential energy with the TOTAL
>>> energy minus the KINETIC energy in my log file. The two are quite
>>> different -- about 370 kcal/mol on average. However the average
>>> total potential energy is about 4460 kcal/mol, so the mean error is
>>> about 8%. Am I doing something wrong here? Or is this to be expected?
>>> At the end of the day what I believe I want is simply the namdEnergy
>>> numbers for selection 1= PROTEIN , selection 2= "everything else",
>>> added to the PROTEIN-PROTEIN namdEnergies. However the errors
>>> present in my sanity check are giving me pause that the namdEnergies
>>> are trustable.
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