[gmx-users] vacuum right but melt collapse

Fri Mar 9 10:07:47 CET 2007

Hi Kitty,

OPLS was developed (IIRC) for use with a cut-off of 1.2 nm. Not
shorter, not longer. If you set the cut-off higher, you introduce
additional force components; noise, artefacts, not precision. It may
be good to do some reading about force fields and the parameterization
thereof.

The reaction field is a correction for the neglect of electrostatic
interaction beyond the cut off. It should be used when using a GROMOS
force field, as those are parameterized for use with it. Regarding
OPLS, I'm not sure. I thought it was not parameterized for use with
RF, but either a plain cut-off or a cut-off with a switch function.
You should check the OPLS papers to be sure.

By the way, please keep discussions on the list... In that way, it is
publicly archived, which potentially saves us a lot of trouble
answering the same questions over and over.

Cheers,

Tsjerk

On 3/9/07, kitty ji <jiqing at iccas.ac.cn> wrote:
> Thanks for your reply.
>
> The force field I used here is OPLS. Increasing the cutoff means to increase the precision of caluculation not "underlying cause". Therefore it is acceptable. In addition, it is said that cutoff 9 angstrom is not enough for electricity calculation.
>
> what is using reaction-field? I thought it was not used here.
>
>
> ----- Original Message -----
> From: "Tsjerk Wassenaar" <tsjerkw at gmail.com>
> To: "kitty ji" <jiqing at iccas.ac.cn>; "Discussion list for GROMACS users" <gmx-users at gromacs.org>
> Sent: Thursday, March 08, 2007 10:34 PM
> Subject: Re: Re: [gmx-users] vacuum right but melt collapse
>
>
> Hi Kitty,
>
> Such a solution is unsatisfactory, as it seems that you're masking an
> underlying cause. Now, I'm not sure of your other parameters, but when
> using a cut-off (do you use reaction-field) you would usually (gromos
> force field) use a twin-range of 0.9/1.4, which is what the force
> field was parameterized for.
>
> If you look at the energies in order to find the cause of a crash, you
> want to look for the energy term first displaying strange behaviour.
> You also may want to try setting the neighbour list update to every
> step.
>
> And please, don't use html formatted e-mails... :)
>
> Hope it helps,
>
> Tsjerk
>
>
> On 3/8/07, kitty ji <jiqing at iccas.ac.cn> wrote:
> >
> >
> >
> >
> > Tsjerk Wassenaar, thank you very much for you suggestion even if most of
> > them have been manipulated.
> >
> >
> >
> > "Write out the energies at every step and check which of the energy
> > contributions goes astray. This may give you a good clue to where
> > you're mistake is. Also, run through the topology again, by hand,
> > checking all bonds, angles, dihedrals, exclusion (implicit ones!),
> > etc."
> >
> > when the explosion happened, the energy of bond, angle and potential will
> > were increase. And the topology has been checked several times.
> >
> >
> >
> >
> >
> > Now I increase the cutoff of VDW & Coulomb from 0.9 to 1.5. The system looks
> > ok from last night to this time. If the problem is fixed, I conclude that
> > cutoff 0.9 is not enough for electric system.
> >
> >
> >
> > Hope this conclusion is helpful for GMX users.
> >
> >
> >
> > Message: 2
> > Date: Wed, 7 Mar 2007 17:49:30 +0100
> > From: "Tsjerk Wassenaar" <tsjerkw at gmail.com>
> > Subject: Re: Re: [gmx-users] vacuum right but melt collapse
> > To: "kitty ji" <jiqing at iccas.ac.cn>, "Discussion list for GROMACS
> > users" <gmx-users at gromacs.org>
> > Message-ID:
> > <8ff898150703070849r59cb8a8dr60b894a881fcc331 at mail.gmail.com>
> > Content-Type: text/plain; charset=GB2312; format=flowed
> >
> >
> > Hi Ji Qing,
> >
> > Write out the energies at every step and check which of the energy
> > contributions goes astray. This may give you a good clue to where
> > you're mistake is. Also, run through the topology again, by hand,
> > checking all bonds, angles, dihedrals, exclusion (implicit ones!),
> > etc.
> >
> > Good luck,
> >
> > Tsjerk
> >
> > On 3/7/07, kitty ji <jiqing at iccas.ac.cn> wrote:
> >
> > >
> > >
> > > Thanks Mark.
> > > But when one polymer chain was used, non-bonded interaction has already
> > been
> > > there. Because one chain means C44H156O22.
> > > After all, I'd like to try more chains.
> > >
> > > Message: 1
> > > Date: Tue, 06 Mar 2007 17:01:52 +1100
> > > From: Mark Abraham <Mark.Abraham at anu.edu.au>
> >
> > > Subject: Re: [gmx-users] vacuum right but melt collapse
> > > To: Discussion list for GROMACS users <gmx-users at gromacs.org>
> > > Message-ID: <45ED03D0.6010803 at anu.edu.au>
> >
> > > Content-Type: text/plain; charset=GB2312
> > >
> > > kitty ji wrote:
> > > > Hi GMX user:
> > > >
> > > > A model for Poly(vinyl methyl ether) was built with OPLS force field (
> > > > all H ).
> > > >
> > > >
> > > >
> > > > When the model was run alone (a single chain) in vacuum, anything goes
> > ok.
> > > >
> > > >
> > > >
> > > > But with 45 chains box, the system will be collapse with a little bond
> > > > elongates abnormal whatever in NVT or NPT.
> > > >
> > > >
> > > >
> > > > The relax time of pressure and temperature were adjust but no helpful.
> > > >
> > > >
> > > >
> > > > How can I found some possible reason and then fix it ? Any advise will
> > > > be appreciated.
> > >
> > > If an isolated molecule doesn't do anything strange, and two more more
> > > do, then you have a problem with inter-molecular interactions, i.e. the
> > > non-bonded ones. Try with just 2 chains to verify this... and look
> > > carefully at the atom types and the non-bonded parameters for them.
> > >
> > > Mark
> > >
> > >
> > >
> > > *************************************************
> > > Ji Qing
> > > Institute of Chemistry, Chinese Academy of Sciences
> > > Tel: 0086-10-62562894  £¬82618423
> > > *************************************************
> > > _______________________________________________
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> >
> >
> > --
> > Tsjerk A. Wassenaar, Ph.D.
> > Junior UD (post-doc)
> > Biomolecular NMR, Bijvoet Center
> > Utrecht University
> > Padualaan 8
> > 3584 CH Utrecht
> > The Netherlands
> > P: +31-30-2539931
> > F: +31-30-2537623
> >
> > _______________________________________________
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>
>
> --
> Tsjerk A. Wassenaar, Ph.D.
> Junior UD (post-doc)
> Biomolecular NMR, Bijvoet Center
> Utrecht University
> Padualaan 8
> 3584 CH Utrecht
> The Netherlands
> P: +31-30-2539931
> F: +31-30-2537623

-- 
Tsjerk A. Wassenaar, Ph.D.
Junior UD (post-doc)
Biomolecular NMR, Bijvoet Center
Utrecht University
Padualaan 8
3584 CH Utrecht
The Netherlands
P: +31-30-2539931
F: +31-30-2537623