[gmx-users] Re: trouble parallelizing a simulation over a cluster

Carsten Kutzner ckutzne at gwdg.de
Wed Dec 8 13:16:38 CET 2010


On Dec 8, 2010, at 1:03 PM, Hassan Shallal wrote:

> Thanks a lot Justin for the very helpful answers concerning the pressure equilibration. Using Berendsen Barostat over 200 ps has lead to the correct average pressure...
>  
> I have another issue to discuss with you and with the Gromacs mailing list members;
>  
> I have been trying to run a simulation on a computer cluster for the first time using a sub file script. What happened is that the .sub file attempted to run the simulation 24 times instead of parallelizing it over the 24 processors!!!!
>  
> Here are the contents of run_1.sub file I tried to use to parallelize the simulation using qsub run_1.sub
>  
> #PBS -S /bin/bash
> #PBS -N run_1
> #PBS -l nodes=3:ppn=8
> module load openmpi/gnu
> mpirun -np 24 /home/hassan/bin/bin/mdrun_mpi -deffnm run_1 -v &> run_1_update.txt
> exit $?
> What happens it that it outputs 24 run_1.log files, starting from #run_1.log1# all the way to #run_1.log23#...Has anyone faced this problem before? and If yes, any hints or solutions?
This typically happens when using a serial mdrun. You should check with ldd whether
mdrun_mpi is linked to the correct mpi library.

Carsten


>  
> I do appreciate any help in that domain
> Hassan
> 
> From: gmx-users-bounces at gromacs.org on behalf of Justin A. Lemkul
> Sent: Mon 12/6/2010 6:43 PM
> To: Discussion list for GROMACS users
> Subject: Re: [gmx-users] pressure fluctuations
> 
> 
> 
> Hassan Shallal wrote:
> > Dear Gromacs users,
> > 
> > I have some concerns about the both the pressure fluctuations and
> > averages I obtained during the equilibration phase. I have already read
> > through several similar posts as well as the following link
> > http://www.gromacs.org/Documentation/Terminology/Pressure. I understand
> > the pressure is a macroscopic rather than instantaneous property and the
> > average is what really matters. I also found out through similar posts
> > that negative average pressure indicates the system tendency to contract.
> > 
> > In the above link, it mentioned that pressure fluctuations should
> > decrease significantly with increasing the system's size. In my cases, I
> > have a fairly big systems (case_1 with *17393* water molecules
> > and case_2 with *11946 *water molecules). However, the pressure still
> > has huge fluctuations (around 500 bars) from the reference value (1
> > bar). Here are the average pressure and density values resulting from
> > the equilibration phases of two cases, please notice the negative
> > average pressure values in both cases...
> > 
> > Case_1_pressure:
> > Energy                      Average   Err.Est.       RMSD  Tot-Drift
> > -------------------------------------------------------------------------------
> > Pressure                   *-2.48342*       0.92    369.709   -4.89668 
> > (bar)
> > Case_1_density:
> > Energy                      Average   Err.Est.       RMSD  Tot-Drift
> > -------------------------------------------------------------------------------
> > Density                     1022.89       0.38     3.8253    2.36724 
> > (kg/m^3)
> > Case_2_pressure:
> > Energy                      Average   Err.Est.       RMSD  Tot-Drift
> > -------------------------------------------------------------------------------
> > Pressure                   *-8.25259*        2.6    423.681   -12.1722 
> > (bar)
> > Case_2_density:
> > Energy                      Average   Err.Est.       RMSD  Tot-Drift
> > -------------------------------------------------------------------------------
> > Density                     1034.11       0.37    2.49964    1.35551 
> > (kg/m^3)
> > 
> > So I have some questions to address my concerns:
> > 1- each of the above systems has a protein molecule, NaCl to give 0.15 M
> > system and solvent (water) molecules... Could that tendency to contract
> > be an artifact of buffering the system with sodium and chloride ions?
> > 
> 
> I suppose anything is possible, but given that these are fairly standard
> conditions for most simulations, I tend to doubt it.  My own (similar) systems
> do not show this problem.
> 
> > 2- how to deal with the tendency of my system to contract?  Should
> > I change the number of water molecules in the system? 
> > or
> > Is it possible to improve the average pressure of the above systems by
> > increasing the time of equilibration from 100 ps to may be 500 ps or
> > even 1 ns?
> > 
> > 3- Is there a widely used range of average pressure (for ref_p = 1 bar)
> > that indicates acceptable equilibration of the system prior to the
> > production?
> > 
> 
> To answer #2 and #3 simultaneously - equilibration is considered "finished" when
> your system stabilizes at the appropriate conditions (usually temperature and
> pressure).  Your results indicate that your equilibrium is insufficient.
> 
> > 4- I can't understand how the system has a tendency to contract whereas
> > the average density of the solvent is already slightly higher than it
> > should be (1000 kg/m^3).
> 
> The contraction causes the density to rise.  Pressure and density are not
> independent; density is a result of pressure.
> 
> > I would like to ignore the pressure based judgement of the above
> > equilibration given that the average density values are very close to
> > the natural value (1000 kg/m^3) (by the way I am using tip3p water model
> > with CHARMM27 ff) Any comment!!
> > 
> 
> It is not guaranteed that simulation water models will reproduce the real
> (experimental) density of water.  If memory serves, the expected density of
> TIP3P should be ~0.98 g mL^{-1}, but I could be wrong.
> 
> > 5- Is the huge fluctuation of the pressure values of the above system
> > despite thier large sizes still acceptable? or large fluctuation is only
> > acceptable for small size systems and is unacceptable for large size
> > systems?
> > If it is unacceptable, any idea of how could it be alleviated or minimized?
> > 
> 
> The fluctuations seem reasonable.  You might try equilibrating with the
> Berendsen barostat first, then switching to Parrinello-Rahman.  The P-R barostat
> allows for wider fluctuations, so if the system is poorly equilibrated, your
> system will be slower to converge.
> 
> -Justin
> 
> > I am including the .mdp used in the above equilibration in case it is
> > needed.
> > 
> > Any feedback or response to the above questions is so much appreciated..
> > 
> > Great regards
> > Hassan
> > 
> > .mdp used for the above equilibration
> > define  = -DPOSRES ; position restrain the protein
> > ; Run parameters
> > integrator = md  ; leap-frog integrator
> > nsteps  = 25000  ; 4 * 25000 = 100 ps
> > dt  = 0.004  ; 4 fs, virtual sites along with heavy hydrogens are used
> > ; Output control
> > nstxout  = 100  ; save coordinates every 0.2 ps
> > nstvout  = 100  ; save velocities every 0.2 ps
> > nstenergy = 100  ; save energies every 0.2 ps
> > nstlog  = 100  ; update log file every 0.2 ps
> > ; Bond parameters
> > continuation = yes  ; Restarting after NVT
> > constraint_algorithm = lincs ; holonomic constraints
> > constraints = all-bonds ; all bonds (even heavy atom-H bonds) constrained
> > lincs_iter = 1  ; accuracy of LINCS
> > lincs_order = 6  ; also related to accuracy, changed to 6 because of
> > using virtual sites along with a larger time step
> > ; Neighborsearching
> > ns_type  = grid  ; search neighboring grid cells
> > nstlist  = 5  ; 20 fs
> > rlist  = 1.2  ; short-range neighborlist cutoff, equal to rcoulomb to
> > allow for PME electrostatics  (in nm)
> > ; Lennard-Jones
> > vdwtype         = switch        ; VDW interactions are switched of
> > between 1 and 1.2
> > rvdw_switch     = 1             ;
> > rvdw            = 1.2           ; short-range vdw cutoff, optimal for
> > CHARMM27 ff  (in nm)
> > ; Electrostatics
> > coulombtype = PME    ; Particle Mesh Ewald for long-range electrostatics
> > pme_order = 4  ; cubic interpolation
> > rcoulomb = 1.2  ; short-range electrostatic cutoff, optimal for CHARMM27
> > ff  (in nm)
> > ; Temperature coupling is on
> > tcoupl  = V-rescale ; modified Berendsen thermostat
> > tc-grps  = Protein Non-Protein ; two coupling groups - more accurate
> > tau_t  = 0.1 0.1 ; time constant, in ps
> > ref_t  = 300  300 ; reference temperature, one for each group, in K
> > ; Pressure coupling is on
> > pcoupl  = Parrinello-Rahman ; Pressure coupling on in NPT
> > pcoupltype = isotropic ; uniform scaling of box vectors
> > tau_p  = 1  ; in ps
> > ref_p  = 1.0  ; reference pressure, in bar
> > compressibility = 4.5e-5 ; isothermal compressibility of water, bar^-1
> > ; Periodic boundary conditions
> > pbc  = xyz  ; 3-D PBC
> > ; Dispersion correction
> > DispCorr = EnerPres ; account for switch vdW scheme
> > ; Velocity generation
> > gen_vel  = no  ; Velocity generation is off
> > 
> >
> 
> --
> ========================================
> 
> Justin A. Lemkul
> Ph.D. Candidate
> ICTAS Doctoral Scholar
> MILES-IGERT Trainee
> Department of Biochemistry
> Virginia Tech
> Blacksburg, VA
> jalemkul[at]vt.edu | (540) 231-9080
> http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin
> 
> ========================================
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