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
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