# [gmx-users] how to calculate ESP at certain position

André Farias de Moura moura at ufscar.br
Wed Aug 5 14:54:31 CEST 2015

```this equation amounts to the double integral of the charge density, it is
one possible physical definition of the electrostatic potential, not a
computational definition. I'd say you need some background reading on the
physics of what you are planning to calculate.

On Tue, Aug 4, 2015 at 11:00 PM, Zhenyu Meng <fdmm1989 at gmail.com> wrote:

> Dear Andre,
> I ask this question because I read a related paper which use point-charge
> potential: Φ=4πεΣqi/ri .
> I'm very curious about how it could fulfill this since they mention they
> use GROMACS but according to my knowledge GROMACS does not have such a
> tool.
> So I suppose they write their own code.
> Here is the link of the paper:
> http://pubs.acs.org/doi/abs/10.1021/jp803661f
>
>
>
> 2015-08-04 21:30 GMT+08:00 André Farias de Moura <moura at ufscar.br>:
>
> > Dear Zhenyu,
> >
> > I would be careful with the choice of words: you cannot say that a point
> in
> > space has an electrostatic potential without saying where you have set
> the
> > reference electrostatic potential to zero (it is a relative quantity, not
> > absolute).
> >
> > I never used the spherical version of g_potential myself, but assuming it
> > does what my lab made code does, you will get a profile of V(r) around
> the
> > COM of the reference group and the value at the COM should always be
> > compared with the values of the surrounding spherical layers, so you will
> > be able to state that the electrostatic potential is either higher or
> lower
> > at that point with respect with a reference position in the radial
> profile
> >
> > Also mind that the COM is not an actual point, you are performing a
> > numerical double integration of the charge density between r and r+dr, so
> > the COM is actually a small sphere. Being small, I do not think that
> > instantaneous calculation of electrostatic potential would make any sense
> > at all, the region around the COM is always the region where the
> potential
> > takes longer to converge (tens of nanoseconds in my case). This is not an
> > artifact of the program: parallel layers along one direction of the box
> > have the same volume and are sampled with the same statistical precision,
> > whereas spherical layers have increasing volumes as we move from the COM,
> > meaning that statistical sampling is very poor at the COM as compared to
> > the outer layers (I would expect wild oscillations without physical
> > significance at the origin of the profile).
> >
> > best
> >
> > Andre
> >
> > On Tue, Aug 4, 2015 at 12:58 AM, Zhenyu Meng <fdmm1989 at gmail.com> wrote:
> >
> > > Dear all,
> > > I want to calculate electrostatic potential at a certain position in a
> > box.
> > > I check the available tools and found only g_potential is related with
> > > this.
> > > However g_potential give the graph of potential vs. box length(may be z
> > > axis).
> > > So my first question: Is this potential value the potential at the COM
> of
> > > the group I select?
> > > Second question: how can I calculate the potential at certain position
> > vs.
> > > time?
> > >
> > > --
> > > Sincerely,
> > > Mr. Meng Zhenyu
> > > Division of Chemistry and Biological Chemistry
> > > School of Physical and Mathematical Sciences
> > > Nanyang Technological University
> > > --
> > > 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.
> > >
> >
> >
> >
> > --
> > _____________
> >
> > Prof. Dr. André Farias de Moura
> > Department of Chemistry
> > Federal University of São Carlos
> > São Carlos - Brazil
> > phone: +55-16-3351-8090
> > --
> > 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.
>
>
>
>
> --
> Sincerely,
> Mr. Meng Zhenyu
> Division of Chemistry and Biological Chemistry
> School of Physical and Mathematical Sciences
> Nanyang Technological University
> --
> 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.
>

--
_____________

Prof. Dr. André Farias de Moura
Department of Chemistry
Federal University of São Carlos
São Carlos - Brazil
phone: +55-16-3351-8090
```