# [gmx-users] Van der waals types

Mark Abraham mark.abraham at anu.edu.au
Wed Apr 11 14:30:28 CEST 2012

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On 11/04/12, Chrisostomos Batistakis <tomyvanbatis at gmail.com> wrote:

> Dear Mark
>
> Many thanks for your responces.
>
> Yes, of course I have seen that. But what I am asking is if when using vdw-type=cut-off  the potential is shifted acoording to the following equation:
>
> VLJ=V(r)-V(rc) for r<rc

No. It's cut off - discontinuous and probably fattening. It's equation 4.23 with r_1 = r_c

Mark

>
>
>
> Regards, Chrysostomos
>
>
> On Wed, Apr 11, 2012 at 1:01 PM, Mark Abraham <mark.abraham at anu.edu.au> wrote:
>
>
> >
> >
> >
> >
> > On 11/04/12, Chrisostomos Batistakis <tomyvanbatis at gmail.com> wrote:
> >
> >
> > > Dear Mark
> > >
> > > I have already checked both sections but I don't understand.. That's why I am asking for help :-)
> >
> >
> >
> > Then you'll have seen that by construction the shifted potential is zero above the cutoff. The question of truncation does not arise.
> >
> >
> > Mark
> >
> >
> >
> >
> >
> > >
> > >
> > > On Wed, Apr 11, 2012 at 12:18 PM, Mark Abraham <mark.abraham at anu.edu.au> wrote:
> > >
> > >
> > > >
> > > >
> > > >
> > > > On 11/04/12, Chrisostomos Batistakis <tomyvanbatis at gmail.com> wrote:
> > > >
> > > > >
> > > > > Dear all
> > > > >
> > > > > I am looking in the manual but I don't understand...
> > > > >
> > > > > When using vdw-type =cut-off in the .mdp file, is the LJ potential only truncated in the cut-off distance rc,  or is it also shifted in order to avoid discontinuity when rij=rc ?
> > > > >
> > > > > If no, how can I have a shifted and truncated LJ potential?
> > > > >
> > > >
> > > >
> > > >
> > > >
> > > > Please check out the manual. Sections 4.1 and 7.3.11 come to mind.
> > > >
> > > >
> > > > Mark
> > > >
> > > > --
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> >
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> >
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