# [gmx-users] Maximum Force constant for Position Restraints

Alex nedomacho at gmail.com
Tue Apr 21 21:40:53 CEST 2015

```No, it does not depend on the system content, aside from the mass of the
particle.
For a simulation requiring numerical integration in time, there is a limit,
and I just estimated it above. For the real world, the limit is that
there's no such thing as a harmonic position restraint. :)
If you want to be really hardcore about it, and still assume there _is_
such a thing as harmonic restraints, then the "timestep" of the real world

Alex

On Tue, Apr 21, 2015 at 1:36 PM, Marcelo Depólo <marcelodepolo at gmail.com>
wrote:

> Thanks Alex. But then, theoretically, is there no limit? All depends on
>
> 2015-04-21 16:16 GMT-03:00 Alex <nedomacho at gmail.com>:
>
> > Correction: tau/pi on the left for the highest value and 5*tau/pi for the
> > 10 x period suggestion above.
> >
> > On Tue, Apr 21, 2015 at 1:13 PM, Alex <nedomacho at gmail.com> wrote:
> >
> > > I think this can be estimated from a general physical argument. The
> > > absolute max in my opinion should come from
> > > 4*pi*tau = sqrt(m/k), where m is the mass of the lightest restrained
> > > particle in the system, k is the constant you seek, and tau is the
> > > timestep.
> > > The coefficient is four because of the Nyquist theorem. I believe GMX
> > > tests for high vibrational frequencies according to 20*pi*tau on the
> left
> > > (ten times the period).
> > >
> > > Hope this helps.
> > >
> > > Alex
> > >
> > >
> > > On Tue, Apr 21, 2015 at 1:02 PM, Marcelo Depólo <
> marcelodepolo at gmail.com
> > >
> > > wrote:
> > >
> > >> Hi guys!
> > >>
> > >> Is there a maximum value of Force Constant for Position restraints?
> > >> the Manual I could only find the default and the equation to describe
> > it.
> > >>
> > >> Best,
> > >> --
> > >> Marcelo Depólo Polêto
> > >> Group of Structural Bioinformatics - Center of Biotechnology
> > >> Student of MSc Cell and Molecular Biology - UFRGS (Brazil)
> > >> B.Sc. Biochemistry - University of Viçosa (Brazil)
> > >> --
> > >> Gromacs Users mailing list
> > >>
> > >> * Please search the archive at
> > >> http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before
> > >> posting!
> > >>
> > >> * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
> > >>
> > >> * For (un)subscribe requests visit
> > >> https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or
> > >> send a mail to gmx-users-request at gromacs.org.
> > >
> > >
> > >
> > --
> > Gromacs Users mailing list
> >
> > * Please search the archive at
> > http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before
> > posting!
> >
> > * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
> >
> > * For (un)subscribe requests visit
> > https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or
> > send a mail to gmx-users-request at gromacs.org.
> >
>
>
>
> --
> Marcelo Depólo Polêto
> Group of Structural Bioinformatics - Center of Biotechnology
> Student of MSc Cell and Molecular Biology - UFRGS (Brazil)
> B.Sc. Biochemistry - University of Viçosa (Brazil)
> --
> Gromacs Users mailing list
>
> * Please search the archive at
> http://www.gromacs.org/Support/Mailing_Lists/GMX-Users_List before
> posting!
>
> * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
>
> * For (un)subscribe requests visit
> https://maillist.sys.kth.se/mailman/listinfo/gromacs.org_gmx-users or
> send a mail to gmx-users-request at gromacs.org.
>
```