Subject: Charge

From: [email-masked] (Bill Ross )
Date: Sun, 2 Oct 1994 13:10:29 -0700

From: "E. Lewars" <[email-masked]>

	It is said that atomic charges in molecules are not observables, ...
	Are charges unobservable/unmeasurable because of fundamental 
	quantum-mechanical reasons  ...  or could an experiment be
	designed *en principe* to give an objective measure of the 
	atomic charges in a molecule?  

One experimental approach is to use electron densities derived 
from X-ray diffraction data to derive point charges. See
David Pearlman and Sung-Hou Kim, "Atomic Charges for DNA
Constituents Derived from Single-crystal X-ray Diffraction
Data," J. Mol. Biol. (1990) 211, 171-187.

No matter how 'objective' the measure of point charges, they
must allow other observables to be reproduced if they are to
be of value in the molecular mechanical context. In practice,
such factors as the convention of only considering pairwise 
interactions that is usually used for speed of calculation 
and the convenience of fixing charges for a range of confomers 
may dictate non-objective fitting.

Bill Ross