Sent to CCL by: Herbert Fruchtl [herbert.fruchtl%%st-andrews.ac.uk]
Intuitively, I would center any multipole analysis at the centre of charge. The centre of atomic (not just nuclear!) charges, calculated in any partition scheme (Mulliken, Hirshfeld, Bader, ...) should be a reasonable approximation of this.
On 08/05/13 12:58, Reis Heribert hreis:_:eie.gr wrote:
Hello John,<http://gmail.com> <owner-chemistry!^!ccl.net
In my opinion the choice of the origin for multipoles is most of the time just a
matter of what one is trying to investigate. So a choice like the one you
propose may be interesting in questions related to, say, chemical reactivity.
But apart from that, it is not quite clear to me how you would go about
computing such an origin. Computing the electron density due to the valence
orbitals on a grid? Or do you have another idea in mind? Maybe something related
to Hirshfelds' free atom densities?
On Tue, May 7, 2013 at 8:53 PM, John McKelvey jmmckel===gmail.com<http://hreis_-_eie.gr> <owner-chemistry/./ccl.net
Perhaps there is another way to look at it, and it is a subset of looking at
the center of nuclear charge. Perhaps it is more sensitive (?) to look at
the center of constituent free atom valence electrons?
On Tue, May 7, 2013 at 12:26 PM, Reis Heribert hreis_-_eie.gr260-489-2160 <tel:260-489-2160>
The definition of the dipole moment of a charged species is well
defined. The only problem with it (and with all higher multipoles) is,
that its value depends on the origin of the coordinate system. Thus for
comparison purposes the origin of the coordinate system of different
molecules should be comparable. A very common choice is the centre of
mass; another possibility is the center of nuclear charge, which is the
standard origin in Gaussian09, if one does not specify the 'Nosymm'
keyword. You can also turn the problem on its head and define the origin
as that point where the dipole is zero...
I'm assuming that you want to compare the absolute value of the dipole
between different molecules. In case you also want to compare the
components, you also should use comparable orientations of the
coordinate systems (e.g the coordinate system where e.g. the inertia (or
polarizability or ...) tensor is diagonal). This, however, is
independent of being charged or uncharged.
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