# [gmx-users] Problems about sigma & epsilon to C6 & C12

Janne Hirvi janne.hirvi at joensuu.fi
Mon Jul 16 09:35:43 CEST 2007

```Hello!

functions so that I combined sigma and epsilon values using rule 3

sigma(ij)=(sigma(i)*sigma(j))^(1/2)
epsilon(ij)=(epsilon(i)*epsilon(j))^(1/2)

and corresponding C6 (4*epsilon*sigma^6) and C12 (4*epsilon*sigma^12) values
using rule 1

C6(ij)=(C6(i)*C6(j))^(1/2)
C12(ij)=(C12(i)*C12(j))^(1/2)

and got identical plots. What I have missed?

Janne

>
> My problem has solved. Finally I realized that my calculation is wrong.
> Thank you so much!
>
> On 7/15/07, james zhang <james.zhangj at gmail.com> wrote:
> >
> > From equation 5.3 page 88, I think rule 1 and 3 are equivalent if you
> > convert the parameters.
> > I am sorry. What's the different between rule 1 and 3?
> > I think C6/C12 are used in rule 1 and sigma/epsilon are used in rule 3.
> > But if you convert sigma/epsilon to C6/C12, these two should be
> equivalent.
> > I am confused. Can I convert sigma/epsilon to C6/C12 if I want to use gmx
> > force field instead of OPLS ?
> > Thanks.
> >
> > On 7/15/07, David van der Spoel < spoel at xray.bmc.uu.se> wrote:
> > >
> > > james zhang wrote:
> > > > I used rule 3 for ffopls rule 1 for ffgmx.
> > > >
> > > please read the manual.these combination rules are not equivalent.
> > >
> > > there is also combination rule 2, see 5.3.3
> > > --
> > > David.
> > > ________________________________________________________________________
> > >
> > > David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
> > > Dept. of Cell and Molecular Biology, Uppsala University.
> > > Husargatan 3, Box 596,          75124 Uppsala, Sweden
> > > phone:  46 18 471 4205          fax: 46 18 511 755
> > > spoel at xray.bmc.uu.se    spoel at gromacs.org   http://folding.bmc.uu.se
> > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >
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```