Subject: Re: CCL:G:Dipole moment of a charged species (Summary)

From: John McKelvey <[email-masked]>
Date: Sat, 18 Jul 1998 10:25:17 -0700

An origin independent dipole  moment of a charged molecule _is not defined_.
The leading term of the multipole moment expansion is Q*Z where Q is the
molecular charge and Z represents a position in space.  If a molecule is
charged then some _arbitrary_ choice has to be made for Z; thus you can
have a dipole moment for a molecule if you are convinced about the location
of Z.

John McKelvey
NCSA

At 04:10 PM 7/16/98 -0700, you wrote:
>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:
>
>***************************************************************************
****
>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?
>
>***************************************************************************
***
>From:    Wayne Steinmetz <[email-masked]>
>Date:    Tue Jul 14, 10:17am -0700
>To:      [email-masked]
>Cc:
>Subject: Re: CCL:G: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.
>
>*****************************************************************************
>From:    [email-masked] <[email-masked]>
>Date:    Tue Jul 14, 10:35am -0700
>To:      [email-masked]
>Cc:      [email-masked]
>Subject: Re:  CCL:G: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
>
>***************************************************************************
>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: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: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
>
>***********************************************************************
>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
>
>**************************************************************************
>
>
>-- 
>Emad
>*********************************************************************
>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] 
>*********************************************************************
>* "Never express yourself more clearly than you think." -Niels Bohr *
>*********************************************************************
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