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Dear gmx-users,<br><br>
I am having trouble getting an mdrun to start successfully. My
system is a periodic box containing 20 polylactide 50-mers of the residue
form SL1-(SL2)n-SL3. Prior to an mdrun, I minimized the system with
cg to Epot = -5.1736e4 and Fmax = 0.979. My grompp command line for
the input to the mdrun includes the *.gro and the *.trr file from the
output of the final minimization. However, when I try an mdrun, the
system behaves as if there are very large forces present: I get LJ (SR)
values of 1e10, "1-4 interactions at a distance greater than 1"
warnings to screen and "1-4 interaction not within cutoff" in
the -debug file, the temperature and pressure increase to the order of
1e18, and everything goes "nan". Also, it says
"Large VCM (group rest):
-0.0000,<x-tab> </x-tab>-0.0000, -0.0000.
How is (negative) zero large?<br><br>
I have tried changing several parameters to see if it would help, to no
avail. Here is a representative mdp file for the simulations after
I had turned off the T and P coupling.<br><br>
<font face="Courier New, Courier" size=2>define
= <br>
integrator
= md<br>
dt
= 0.0005 ;ps<br>
nsteps
= 10000 ; <br>
nstcomm<x-tab> </x-tab><x-tab> </x-tab>
= +1<br>
nstxout
= 5000<br>
nstvout
= 5000<br>
nstlog
= 5000<br>
nstenergy
= 5000<br>
nstfout<x-tab> </x-tab><x-tab> </x-tab>
= 5000<br>
nstxtcout
= 5000<br>
;xtc_grps
=
SL1<x-tab> </x-tab>SL2<x-tab> </x-tab>SL3<br>
energygrps
= SL1 SL2 SL3<br>
nstlist
= 5<br>
ns_type
= grid<br>
pbc<x-tab> </x-tab><x-tab> </x-tab><x-tab> </x-tab>
= xyz<br>
rlist
= 1.0<br>
coulombtype
= cut-off<br>
rcoulomb
= 1.0<br>
vdwtype<x-tab> </x-tab><x-tab> </x-tab>
= cut-off<br>
rvdw
= 1.0<br>
tcoupl
= no;berendsen<br>
tc-grps
= SL1 SL2 SL3<br>
tau_t
= 0.1 0.1 0.1<br>
ref_t
= 300 300 300<br>
Pcoupl
= no;berendsen<br>
pcoupltype<x-tab> </x-tab><x-tab> </x-tab>
= isotropic <br>
tau_p
= 2.0<br>
compressibility =
4.5e-5<br>
ref_p
= 1.0<br>
gen_vel
= yes<br>
gen_temp
= 50<br>
gen_seed
= 173529<br>
constraints
= none<br><br>
</font>The box is a 20 nm cube, so the molecules aren't seeing their
duplicates. <br><br>
A few somewhat related topics that this question brought to
mind:<br><br>
1. I don't think I really need to couple or record separately for each
residue. How do I declare this in the mdp? With
"system", "Protein", or something else?<br><br>
2. Although the pdb file used to make the system includes 20 distinct
polymer chains, the *.top file lists only a single molecule
(Protein). Is this problematic? Pdb2gmx wouldn't run if I
didn't remove the MODEL-TER-ENDMDL lines from the pdb file. With
smaller systems, I can work around this with chain identifiers, but that
doesn't work so well when the size and number of polymers gets large, and
I would rather have a single *.top file than multiple *.itp
files.<br><br>
My apologies for the jumble of questions, and I look forward to any input
people might have.<br><br>
Thank you,<br><br>
Chris O'Brien<br>
<x-sigsep><p></x-sigsep>
Department of Chemical Engineering<br>
Clemson University<br>
Clemson, SC 29634-0909<br>
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