Subject: CHARGES IN MOLECULES: A QUESTION

From: "E. Lewars" <[email-masked]>
Date: Sun, 2 Oct 1994 13:22:18 -0400

1994 Oct 2

It is said that atomic charges in molecules are not observables, and so there
is no single right way to calculate them.  Hence the fact we have several
(many?) schemes, all of them giving different "answers".

Are charges unobservable/unmeasurable because of fundamental quantum-mechanical
reasons (e.g. there is no QM charge operator, in contrast to there being QM
operators for, say, energy or dipole moment), or could an experiment be
designed *en principe* to give an objective measure of the atomic charges in
a molecule?  When van't Hoff and Le Bell proposed shapes for molecules in the
1870's, these shapes were unobservables, but, we now know, not for fundamental
reasons.

---

From: John Sichel <[email-masked]>
Date: Sun, 2 Oct 1994 16:29:13 -0300 (ADT)

Atomic charges are not well-defined because atoms in molecules are not
well-defined. The QM charge operator is just the charge (= -e for
electrons), so one can integrate -e x (psi)^2 over a volume which
represents the atom, but this volume is not uniquely defined. 

Some quantum chemists use the method proposed by Richard Bader (McMaster
University, Canada) which is based on analysis of the charge density. If
this method is accepted, it does provide a unique partition of the 
electronic charge appropriate for Hartree-Fock quality wavefunctions. 

John Sichel
Universite de Moncton
Moncton, NB, Canada

On Sun, 2 Oct 1994, E. Lewars wrote:

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