molecular dipoles of charged molecules

[cavallo]

Hi Axel,

dipole moments for charged species are indeed position dependent. If
you approximate the charge distribution as sum of various multipoles,
only the first non vanishing multipole is position independent. In
neutral molecules the net charge (zeroth-order multipole) is zero so
that the dipole (if not vanishing) is position independent. In
molecules with zero charge and dipole, such as benzene, the quadrupole
is the first non-zero multipole, and it is position independent.
Thus, in charged molecules the total charge is position independent,
but higher multipoles, including the dipole, do not.

As far as I remember, in charged molecules it is "standard" to
calculate the dipole with reference to the center of mass, while
calculating it at the "center of charges"  should result in a
vanishing dipole.

More on this, including some refs,  at
http://www.ccl.net/chemistry/resources/messages/1995/01/13.002-dir/index.html

Ciao,
Luigi


On Oct 19, 3:32 am, Axel <akoh... at gmail.com> wrote:
> hi,
>
> this is a bit of a badly conditioned problem, but i noticed
> that the molecular dipole value computed for charged molecules
> does not take out any net charge and thus makes the resulting
> dipole strongly position dependent, and thus practically useless.
>
> i've met the same problem while writing a visualization tool for
> VMD and ultimately decided to remove the total charge at the
> center of the molecule to retain position independent results
> (and visualizations).
>
> are there any opinions on this matter?
> or even examples for where the current
> implementation is useful.
>
> thanks,
>     axel.