# [gmx-users] g_dipole ? => dipole moment ?

Chih-Ying Lin chihying2008 at gmail.com
Tue Oct 19 02:01:17 CEST 2010

```Hi

>From David =>
"dipole is 48.0 sum of q_i x_i, therefore if you
have large charge separation you will get a large dipole."

1. What does the coefficient 48.0 represent?
2. Is q_i = partial charge or charge on the atoms / united atoms?
3. Is x_i = the distance between the mass center of atom from the mass
center of the molecule?
4. What does the large charge separation mean?  Do you mean the charged
molecule?  Or, Do you mean the molecule with a long carbon chain?

Thank you
Lin

On 2010-10-18 03.30, Chih-Ying Lin wrote:
> HI
> I confined one molecule in the center of box and issue the g_dipole
command.
> The average dipole moment is still around 32.
> It is the molecule with 33 atoms / united atoms of most carbon groups,
> isn't the dipole moment around 32 too high?
> How can I test next  and  know that the dipole moment around 32 is
> acceptable?
By calculating on paper: dipole is 48.0 sum of q_i x_i, therefore if you
have large charge separation you will get a large dipole.

> Thank you
> Lin
> On 2010-10-16 21.36, Chih-Ying Lin wrote:
>  >
>  > Hi
>  > I issue the g_dipole command on Gromacs => And, the following
>  > information is shown.
>  > There are 10 molecules in the selection,
>  > Does the Average =32.1611 refer to the average for a single over the
>  > simulation time?
>  > Or, the Average = 32.1611 summing for all the 10 molecules over the
>  > simulation time?
>  > If the average = 32.1611 for a single molecule with 33 atoms / united
>  > atoms of most carbon groups, isn't the dipole moment too high?
> I think this is the average per molecule. You may need to run trjconv
> -pbc whole, because mdrun may break molecules in two parts, meaning that
> the molecule becomes as big as the box.
>
>  >
>  >
>  >
>  >
>  > What does "will subtract their charge at their center of mass"  this
> mean?
>  > Why "will subtract their charge at their center of mass"  ?
>  >
>  >
>  >
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://maillist.sys.kth.se/pipermail/gromacs.org_gmx-users/attachments/20101018/49a536c7/attachment.html>
```