Notes
A superficial picture of long-range electronic interactions envisions
effective "atomic point charges" Q_{A} at each nuclear center
interacting according to the classical potential energy of Coulomb electrostatics (CE):
E_{CE} = Σ_{A,B}
Q_{A}
Q_{B}/R_{AB}
Although totally inadequate in the domain of exchange interactions, this formula
may provide useful estimates of "electrostatic effects" at
longer range (R_{AB} > sum of atomic van der Waals radii). When
evaluated with natural atomic charges, the formula defines what may
be called the
"Natural Coulomb Electrostatics" (E_{NCE}) potential energy
for the species and geometry in question. Variations of E_{NCE}
with respect to geometry changes provide simple estimates of electrostatic
contributions to intra- or intermolecular interaction energy,
supplementing independent estimates of steric
and donor-acceptor contributions for
comparison with more detailed "energy component" analyses
(e.g., NEDA).
From the known NBO charge distributions in the idealized NLS,
one can also evaluate separate Lewis (L) and non-Lewis (NL) contributions to
NPA charge, together with corresponding L/NL contributions
to E_{NCE} that allow estimates
of non-classical NL-induced (resonance type)
charge shifts on overall dielectric properties. These NL-induced
charge shifts are often found to be leading contributors to
chemically important "electrostatic
effects", providing an important quantal correction
to quasi-classical interpretation of total E_{NCE}. For open shells,
an additional
spin-charge NCE table shows the distinct
α-NCE and β-NCE contributions of each spin set, again emphasizing
the limitations of naive classical interpretations.
NCE keyword results are illustrated
for a model water dimer at
RHF/6-311++G** level , showing intra- and intermolecular
NCE values between the two monomer units.
(see NBO 6.0 Manual,
pp. B?-? for additional discussion).