[gmx-users] Hard Spheres

[Sascha Hempel]

On Thu, 2010-12-09 at 09:11 +0100, Berk Hess wrote:
> 
> 
> > Subject: Re: [gmx-users] Hard Spheres
> > From: sascha.hempel at bci.tu-dortmund.de
> > To: gmx-users at gromacs.org
> > Date: Thu, 9 Dec 2010 08:56:54 +0100
> > 
> > > On Wed, Dec 8, 2010 at 4:01 AM, Sascha Hempel
> > > <sascha.hempel at bci.tu-dortmund.de> wrote:
> > > Hi all!
> > > 
> > > I am trying to add some hard spheres to my simulation. As far
> > > as i can
> > > tell from the manual Gromacs supports only LJ or Buckingham
> > > for
> > > non-bonded interaction.
> > > 
> > > 
> > > Why cant you use LJ? By setting the attractive part equal to zero.
> > > 
> > The LJ Equation is 4 * eps * ( (sig/r)**12 - (sig/r)**6).
> > If I either set sigma or epsilon to zero the Potential will become
> zero
> > for all r
> 
> Search in the manual for combination rule 1, then you can set C6 and
> C12.
> Also I just replied a week ago to a mail where a user wanted to set C6
> to zero while supplying sigma and epsilon, you can do that by putting
> a minus sign in front of the sigma.
> 
> Another option is using tabulated potentials (user), then you can use
> anything,
> except for completely hard spheres, which would lead to infinite
> forces.
> 
> Berk
> 

Thanks for your advice. I looked this up in the manual and experimented
a little with the functions. 
I can not just set C6 to 0 beacause then i will keep a repulsing
potential at long distances which will skrew with my system.
I could use a shift or switch function to take care of that, but since i
want some regular water in my system too this is not an option. 

So I am going to go with tabulated potentials. 
How "soft" do my hard spheres have to be to not cause any trouble for
the gromacs engine?

Sascha



> > > 
> > > Is there a way to add hard spheres to lets say a simulation of
> > > Water
> > > molecules without interfering into the regular water-water
> > > interaction?
> > > 
> > > Thanks a lot in advance,
> > > 
> > > Sascha
> > > 
> > > TU Dortmund
> > > Laboratory of Thermodynamics
> > > --
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