Abstract
An atomic version of the Millikan oil drop experiment is performed computationally. It is shown that for planar molecules, the atomic version of the Millikan experiment can be used to define an atomic partial charge that is free from charge flow contributions. We refer to this charge as the Millikan-Thomson (MT) charge. Since the MT charge is directly proportional to the atomic forces under a uniform electric field, it is the most relevant charge for force field developments. The MT charge shows good stability with respect to different choices of the basis set. In addition, the MT charge can be easily calculated even at post-Hartree-Fock levels of theory. With the MT charge, it is shown that for a planar water dimer, the charge transfer from the proton acceptor to the proton donor is about −0.052 e. While both planar hydrated cations and anions show signs of charge transfer, anions show a much more significant charge transfer to the hydration water than the corresponding cations. It might be important to explicitly model the ion charge transfer to water in a force field at least for the anions.
Highlights
Electrons show marked quantum behavior and are always delocalized in space
It has been shown that for a planar molecule, the electron density is not perturbed by any infinitesimal displacement of any atoms in the out-of-plane direction
The procedure can be considered as an atomic version of the Millikan oil drop experiment
Summary
Electrons show marked quantum behavior and are always delocalized in space. Their delocalized distribution along with the fact that electrons are indistinguishable particles with Fermion symmetry makes unique assignments of electrons to atoms in a molecule challenging if not impossible. There would be no charge flow with respect to any infinitesimal displacement of any atom normal to the plane This no-charge-flow condition leads to Eq (3) being exact for planar molecules in a uniform field. We will restrict ourselves mostly to planar molecules, which are completely free of charge flow contributions Another similar definition is the Generalized Atomic Polar Tensor (GAPT) charge, which was defined by using the full trace of the dipole derivate tensor.. We want to emphasize that the MT charges can be determined with any existing electronic structure code as long as the code supports evaluating energies under an external field This is a huge convenience in terms of implementation compared with many other population analysis methods.
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