Numerical Study on the Partitioning of the Molecular Polarizability
into Fluctuating Charge and Induced Atomic Dipole Contributions
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Abstract
In order to carry out a detailed
analysis of the molecular static polarizability, which is the response
of the molecule to a uniform external electric field, the molecular
polarizability was computed using the finite-difference method for
21 small molecules, using density functional theory. Within nine charge
population schemes (Löwdin, Mulliken, Becke, Hirshfeld, CM5,
Hirshfeld-I, NPA, CHELPG, MK-ESP) in common use, the charge fluctuation
contribution is found to dominate the molecular polarizability, with
its ratio ranging from 59.9% with the Hirshfeld or CM5 scheme to 96.2%
with the Mulliken scheme. The Hirshfeld-I scheme is also used to compute
the other contribution to the molecular polarizability coming from
the induced atomic dipoles, and the atomic polarizabilities in eight
small molecules and water pentamer are found to be highly anisotropic
for most atoms. Overall, the results suggest that (a) more emphasis
probably should be placed on the charge fluctuation terms in future
polarizable force field development and (b) an anisotropic polarizability
might be more suitable than an isotropic one in polarizable force
fields based entirely or partially on the induced atomic dipoles