Subject: Dipole moment of a charged species (Summary)

From: "Emadeddin Tajkhorshid" <[email-masked]>
Date: Thu, 16 Jul 1998 16:10:14 -0700

Thank you very much for your replies.

However, it is still not clear to me whether I can report the dipole moments of
two charged species in a scientific paper or not. I mean, can I compare dipole
moments of molecule_1 and molecule_2 (both negative ions, and the dipole
moments are calculated after transforming the coordinate to a mass-weighted or
charge-wieghted system) and can I say that the dipole moment of 1 is larger
than 2, or if we align two molecules, the direction of the dipole vector with
respect to the carbon chain is the opposite for molecule 1 as compared to the
one in molecule 2. Since I would like to report these numbers in a scientific
papers, I am a bit conservative. Perhaps I may pose the question in another
way, asking:
Do you know any paper where the dipole moments of the ionic species are
compared and/or discussed? Or the people prefer not to discuss these numbers in
their paper ....

The Q and summary of replies:

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Dear Colleagues

I have calculated two molecules using gas phase calculations and a number of
different solvent models. They are both negatively charged species. Now the
question is whether and how we can compare the dipole moments between these
molecules and among different solvent models. I mean, the values, and also the
directions, calculated for the dipole moment of a charged species are
completely dependent on the origin of the coordinates. With this regard, how
are the dipole moments calculated in quantom chemical calculations
(particularly, I am interested in the reported values in the output of the
Gaussian94 or DMOL programs)? And (how) can we use them for further analysis of
the results or it is better to look at other properties like the atomic
charges?

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From:    Wayne Steinmetz <[email-masked]>
Date:    Tue Jul 14, 10:17am -0700
To:      [email-masked]
Cc:
Subject: Re: CCL:G:Dipole moment of a charged species

Be very careful since the species to be compared possess net charge.
It is a consequence of the principles of electricity and magnetism that
the dipole moment is dependent on the choice of the origin when the
molecule has net charge.  In fact, the dipole moment can be
made zero by choosing the origin of the charge distribution at the center
of the charge.  The value of the dipole moment is independent of the origin
when the molecule has no net charge.

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From:    [email-masked] <[email-masked]>
Date:    Tue Jul 14, 10:35am -0700
To:      [email-masked]
Cc:      [email-masked]
Subject: Re:  CCL:G:Dipole moment of a charged species

Dear Emad,

For an N point charge system of total charge Q, the dipole moment is

D = sum_i [ x_i * q_i ]

where x and q are position vector and charge on point i.  By translating
to the center of geometry,

X = (1/N) sum_i [ x_i ]

one obtains

D = sum_i [ (x_i - X) * q_i ] + X * sum_i [ q_i ]
  = D0 + X*Q

D0 is a value that is independent of the positions of the point charge and
therefore may be used for comparison purposes.  For a continuous charge
distribution, substitute an integral for the sum.

Regards,
Liang

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From:    Doug Fox <lorentzian.com!gaussian.com!fox>
Date:    Tue Jul 14, 12:19pm -0400
To:      uunet.uu.net!uunet!DKFZ-Heidelberg.de!E.Tajkhorshid%lorentzian.com
Cc:
Subject: Re: CCL:G:Dipole moment of a charged species

  Emad,

   The dipole moment of a charge species includes a contribution from
the separation of charge and the center of nuclear charge.  Thus one
approach is to compare dipoles with the origin at the center of charge
to zero this contribution.  In Gaussian this is the origin for the
standard orientation and the orientation used in multipole report in the
log file, not in the archive entry or if you use NoSymm.  I don't know
what DMOL reports but it should be fairly simple to correct one to the
other.

****************************************************************************
From:    John McKelvey <[email-masked]>
Date:    Tue Jul 14,  2:06pm -0700
To:      [email-masked]
Cc:
Subject: Re: CCL:G:Dipole moment of a charged species

Emad,

I like using the charge-reference-coordinate as being the neutral molecule
valence-electron-weighted (x,y,z) point in space.  It puts the emphasis on
where the meaningful chemistry is.

Mass-weighted systems might also be useful, but there could be a
significant shift, for example, if a H or F atom were replaced by, say,
Iodine.

Regards,

John McKelvey
NCSA

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From:    Preston J. MacDougall <[email-masked]>
Date:    Tue Jul 14,  1:42pm -0700
To:      [email-masked]
Cc:
Subject: atomic first moments

Dr. Tajkhorshid,
   You are correct that the dipole moment of a charged species is
origin dependent.  Packages must select origins anyway, so the computed
dipole moment must be taken for what it is worth.  You can assign
first moments to the consituent atoms in a molecule or ion, whether
there are partial charges on them or not (see J. Chem. Phys., vol. 87,
p. 1142 (1987)).  The first moment of an atom is also origin dependent,
but is naturally chosen as the nucleus.  I am not sure, but I believe
that these first moments can be calculated from a G94 wavefunction using
the AIMPAC option.  If not, the AIMPAC software can be downloaded off of
the internet at http://www.chemistry.mcmaster.ca/aimpac/

Hope this helps in your research,

Dr. Preston J. MacDougall
NASA-ASEE-Stanford Summer Faculty Fellow
MS T27-A, NASA Ames Research Center
Moffett Field, CA 94035

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-- 
Emad
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E. Tajkhorshid				
German Cancer Research Center; DKFZ             Tel: +49 6221 42 2340
Dept. Molecular Biophysics (H0200)              FAX: +49 6221 42 2333
P.O.Box 101949			   
69009 Heidelberg, Germany     Email: [email-masked] 
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* "Never express yourself more clearly than you think." -Niels Bohr *
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