[gmx-users] Positive Coul. recip. term

[Denny Frost]

Thank you both for your replies.  I currently have another ionic liquid
running just fine on the same gromacs build (compiled the tpr file
yesterday), so I am reluctant to conclude that the problem is with the
linking.  Please let me know if you disagree.
The force field I am using was published in 2004 and has been validated by
another group.  I have double and triple checked my itp files to make sure
they match the force field, but it's possible there are still some errors
there.
I agree that this is very strange and feel that there must be something
fundamentally wrong in the mdp file or deeper.  I have included my mdp file
below.

title               =  PMP+TFN
cpp                 =  /lib/cpp
constraints         =  hbonds
integrator          =  md
dt                  =  0.002   ; ps !
nsteps              =  10000000   ; total 20 ns
nstcomm             =  10
nstxout             =  50000
nstvout             =  50000
nstfout             =  0
nstlog              =  5000
nstenergy           =  5000
nstxtcout           =  25000
nstlist             =  10
ns_type             =  grid
pbc                 =  xyz
coulombtype         =  PME
vdwtype             =  Cut-off
rlist               =  1.2
rcoulomb            =  1.2
rvdw                =  1.2
fourierspacing      =  0.12
pme_order           =  4
ewald_rtol          =  1e-5
; Berendsen temperature coupling is on in two groups
Tcoupl              =  v-rescale
tc_grps             =  PMP   TFN
tau_t               =  0.2  0.2
ref_t               =  300  300
nsttcouple          =  1
; Energy monitoring
energygrps          =  PMP   TFN
; Isotropic pressure coupling is now on
Pcoupl              =  berendsen
pcoupltype          =  isotropic
tau_p               =  2.0
ref_p               =  1.0
compressibility     =  4.5e-5

; Generate velocites is on at 300 K.
gen_vel             =  yes
gen_temp            =  300.0
gen_seed            =  -1


On Fri, Feb 24, 2012 at 12:17 AM, Dommert Florian <
dommert at icp.uni-stuttgart.de> wrote:

> On Thu, 2012-02-23 at 13:35 -0700, Denny Frost wrote:
> > Dear all,
> > I am trying to equilibrate a solvent of pure ionic liquid.  The system
> > keeps exploding (after 2-5 ns) and I am not sure why, though I believe
> > coulombic interactions are to blame.  This is because the Coul-SR term
> > is negative, but the Coul. recip term is very positive throughout the
> > entire run (giving the entire system a positive potential energy).  I
> > think this means that the short-range electrostatics are okay, but the
> > long range electrostatics (calculated with PME) are not.  Does anybody
> > have any suggestions as to why this would happen?  I have used the
> > exact same PME input parameters for another ionic liquid that works
> > just fine.  They are listed below.
> >
> >
> > rcoulomb = 1.2
> > fourierspacing = 0.12
> > pme_order = 4
> > ewald_rtol = 1e-5
> >
>
> Depends on the force field you are using. Perhaps it ist not validated
> for the ionic liquid you want to study. It is especially strange, that
> it takes so long for your system to blow up.
>
> Moreoever I would try to optimize the PME settings with the tools,
> g_tune_pme and g_pme_error, which give you performance and accuracy of
> the parameters, respectively.
>
> So perhaps with some more information I can provide more help.
>
> /Flo
>
> >
> > Thanks!
> > Denzil Frost
> > --
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> --
> Florian Dommert
> Dipl. - Phys.
>
> Institute for Computational Physics
> University Stuttgart
>
> Pfaffenwaldring 27
> 70569 Stuttgart
>
> EMail: dommert at icp.uni-stuttgart.de
> Homepage: http://www.icp.uni-stuttgart.de/~icp/Florian_Dommert
>
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>
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