Scratch what I said last night about pressure coupling - it crashes
after ~200K timesteps due to fluctuations in the cell volume. I'm
sticking with NVT for now.
On 5/31/2011 6:14 PM, Michael Daily wrote:
The following mdp file produces a successful dynamics run out to 100K
steps / 200 ps. What I discovered is this:
Using the md integrator, it is necessary to turn off pressure
coupling. However, pressure coupling works with sd (Langevin)
integrator.
Mike
---
; title and include files
title = 1EX6-S35P_md1
;cpp = cpp
;include =
-I/home/yoo2/myusr/gromacs-3.3.3/share/gromacs/top/ -I./
define =
; integrator and input/output setting up
;integrator = md
integrator = sd
nsteps = 1000000 ; 2 ns
dt = 0.002
nstxout = 5000
nstvout = 5000
nstenergy = 500
nstxtcout = 500
nstlog = 500
xtc_grps = System
energygrps = System
comm_mode = Linear
;implicit solvent
implicit_solvent = GBSA
gb_algorithm = OBC
gb_saltconc = 0.15 ; note - this feature is not functional yet
rgbradii = 2.0 ; need larger cut-off than atomistic models
; neighbor searching and vdw/pme setting up
nstlist = 10
ns_type = grid
pbc = xyz
;rlist = 1.4
rlist = 2.0
;coulombtype = pme
coulombtype = Cut-Off
fourierspacing = 0.1
pme_order = 6
;rcoulomb = 1.4
rcoulomb = 2.0
;vdwtype = switch
vdwtype = Cut-Off
rvdw_switch = 1.0
;rvdw = 1.2
rvdw = 2.0
; cpt control
tcoupl = v-rescale
tc-grps = System
tau_t = 0.4
ref_t = 300.0
Pcoupl = parrinello-rahman
pcoupltype = isotropic
tau_p = 1.0
compressibility = 4.5e-5
ref_p = 1.0
; velocity & temperature control
gen_vel = yes
gen_temp = 300.0
annealing = no
constraints = hbonds
constraint_algorithm = lincs
morse = no
---
On 5/30/11 8:45 PM, Michael D. Daily wrote:
Thanks for the recommendations everyone. I tried all of the mdp
changes recommended by Justin (increase rlist, rvdw, etc to 2.0;
change T-coupling to v-scale, and eliminate P-coupling). When I
increased the distance cutoffs, it ran about 30 ps then crashed
instead of crashing immediately. Only when I also turned off
P-coupling did it keep running long-term (now it's up to 500K+ steps
/ 1 ns), so apparently the problem was a lack of control of system
volume. Reducing the timestep from 2 fs to 1fs did not work by
itself and was not necessary ultimately to get it to run.
Josh, thanks for the physical explanation of why the protein might
explode. My starting structure was a minimized xtal structure so I
wouldn't call it "extended." I'll try again with 4.5.4 before I
scale this up any further.
Mike
On 5/30/2011 8:02 PM, Mark Abraham wrote:
On 31/05/2011 10:54 AM, Justin A. Lemkul wrote:
Joshua L. Phillips wrote:
I've found that I often get LINCS warnings like this when starting
from
highly extended conformations when using implicit solvent. The GBSA
surface tension combined with the lack of viscosity (due to the
absence
of explicit water) allows the protein to change conformation much
faster
than LINCS likes by default.
Normally, I just run a short vacuum simulation (keep the same
settings
as Justin suggested but set GBSA = No) to let the system relax a
little
more before starting a GBSA run (EM is often just not enough).
Usually
only about 50ps. Also, doing this with position restraints can
help slow
down the collapse, and usually results in just enough collapse
that the
following GBSA run will satisfy the default LINCS settings.
Another option might be to run a short simulation with GBSA turned
on,
but using Langevin dynamics to include some additional friction that
should slow down the initial collapse.
That's generally a good idea. We often use the sd integrator when
doing GBSA calculations.
Also, you could fudge with the environment variables associated with
LINCS, but this seems a little dangerous compared to the above two
suggestions.
I wouldn't say "a little dangerous," I'd say "very dangerous" :)
If the constraints are failing, it's not necessarily (or usually)
their fault. The system is unstable, and trying to just override
this will give you a trajectory, but you should be concerned that
this trajectory is being ruled by spurious forces.
I would be interested in peoples' opinions and suggestions on how to
handle this issue as it is quite common when starting from highly
extended structures using GBSA. Some proteins are more susceptible
than
others...
Reducing the timestep is a good option. If the dynamics are
occurring so fast that the constraints can't keep up, reducing dt
can help.
Agreed. Even using all-vs-all kernels, I've observed similar
symptoms as the OP lately with excessive rotation of v-site terminal
methyl groups. I can generate stable trajectories by starting my
equilibration with 0.5 fs timesteps, and switching later.
Mark
-Justin
I also tend to find that version 4.5.4 is much more stable than even
4.5.3 for implicit solvent simulations.
-- Josh
On Mon, 2011-05-30 at 18:06 -0500, Michael D. Daily wrote:
Thanks Justin, this is very helpful. I'll attempt these fixes
tomorrow.
Mike
On 5/30/2011 5:50 PM, Justin A. Lemkul wrote:
Michael D. Daily wrote:
Hi all,
I'm trying to run implicit solvent calculations in gromacs 4.5
with the charmm forcefield. I am able to minimize successfully
and compile for
When troubleshooting, it is always advisable to try the latest
version (4.5.4) to see if the problem is reproducible. If a
pertinent bug has been fixed, there's no use troubleshooting the
broken version.
mdrun, but soon after starting, mdrun complains about excessive
rotation in LINCS (see the error printed below that). I also
include my mdp file at the bottom. Can anyone advise me as to
the possible cause of such errors, as it is difficult to
diagnose given that grompp worked fine.
For reference:
http://www.gromacs.org/Documentation/Terminology/Blowing_Up#Diagnosing_an_Unstable_System
If grompp worked, that just means your coordinate and topology
matched and there were no internal conflicts within the .mdp
file. It is no guarantee that the resulting simulation will
actually work, unfortunately.
--- lincs error ---
Step 1, time 0.002 (ps) LINCS WARNING
Typically an instant LINCS failure indicates insufficient
minimization. You said you minimized successfully, but what
does this mean? What values did you achieve for Fmax and Epot?
relative constraint deviation after LINCS:
rms 0.000780, max 0.020692 (between atoms 880 and 881)
bonds that rotated more than 30 degrees:
atom 1 atom 2 angle previous, current, constraint length
606 607 36.7 1.0527 0.1080 0.1080
614 615 35.6 0.8972 0.1125 0.1111
614 616 75.3 0.1054 0.1121 0.1111
880 881 58.0 0.1068 0.1134 0.1111
880 882 50.4 0.9168 0.1121 0.1111
889 890 55.3 0.1066 0.1122 0.1111
889 891 35.3 0.8588 0.1118 0.1111
---- mdp file ------------
; title and include files
title = 1EX6-S35P_md1
cpp = cpp
include =
-I/home/yoo2/myusr/gromacs-3.3.3/share/gromacs/top/ -I./
Any reason you're including files from an ancient Gromacs version?
define =
; integrator and input/output setting up
integrator = md
nsteps = 1000000 ; 2 ns
;nsteps = 5000 ; 2 ns
dt = 0.002
nstxout = 5000
nstvout = 5000
nstenergy = 500
nstxtcout = 500
nstlog = 500
xtc_grps = System
energygrps = System
comm_mode = Linear
;implicit solvent
implicit_solvent = GBSA
gb_algorithm = Still
gb_saltconc = 0.15
FYI, gb_saltconc is nonfunctional. Don't expect it to do
anything :)
rgbradii = 1.0
; neighbor searching and vdw/pme setting up
nstlist = 10
ns_type = grid
pbc = xyz
;rlist = 1.4
rlist = 1.0
;coulombtype = pme
coulombtype = Cut-Off
fourierspacing = 0.1
pme_order = 6
;rcoulomb = 1.4
rcoulomb = 1.1
;vdwtype = switch
vdwtype = Cut-Off
rvdw_switch = 1.0
;rvdw = 1.2
rvdw = 1.1
All of these are potentially problematic. Running implicit
simulations typically requires longer cutoffs than would
normally be needed for explicit solvent simulations. Try
rlist=rvdw=rcoulomb=2.0 nm.
; cpt control
tcoupl = nose-hoover
A better choice for initial equilibration would be either
V-rescale or Berendsen. I know this can be an issue in explicit
solvent, when velocities can oscillate a lot at the outset of a
simulation using Nose-Hoover and the simulation box can explode;
I don't know if this is such a big deal with implicit, but it
can't hurt to try.
tc-grps = System
tau_t = 0.4
ref_t = 300.0
Pcoupl = parrinello-rahman
I don't know how an implicit box will respond to pressure
coupling, but it would be better to try NVT first and see if
it's stable, then try NPT and see if things break down.
One option that might be advantageous is to use the all-vs-all
kernels for a speed upgrade. You can accomplish this with:
rlist = 0
nstlist = 0
rvdw = 0
rcoulomb = 0
rgbradii = 0
pbc = no
comm-mode = angular
You'd have to run with mdrun -pd (particle decomposition), but
the end result can be quite fast and you avoid potential
periodicity effects.
-Justin
--
Michael D. Daily, Ph.D.
Postdoctoral Fellow
Qiang Cui group
Department of Chemistry
University of Wisconsin-Madison
--
Michael D. Daily, Ph.D.
Postdoctoral Fellow
Qiang Cui group
Department of Chemistry
University of Wisconsin-Madison
--
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