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J Chem Theory Comput. 2013 Nov 12;9(11):4779-88. doi: 10.1021/ct4005454. Epub 2013 Oct 1.

Atomic Charges and the Electrostatic Potential Are Ill-Defined in Degenerate Ground States.

Author information

1
Department of Inorganic and Physical Chemistry, Ghent University , Krijgslaan 281 (S3), 9000 Gent, Belgium.
2
Departamento de FĂ­sica, Facultad de Ciencias, Universidad de Chile , Casilla 653, Santiago, Chile.
3
Centro para el desarrollo de la Nanociencias y Nanotecnologia, CEDENNA , Av. Ecuador 3493, Santiago, Chile.
4
Department of Chemistry & Chemical Biology, McMaster University , Hamilton, Ontario, Canada L8S 4M1.

Abstract

A system in a spatially degenerate ground state responds in a qualitatively different way to positive and negative point charges. This means that the molecular electrostatic potential is ill-defined for degenerate ground states due to the ill-defined nature of the electron density. It also means that it is impossible, in practice, to define fixed atomic charges for molecular mechanics simulations of molecules with (quasi-)degenerate ground states. Atomic-polarizability-based models and electronegativity-equalization-type models for molecular polarization also fail to capture this effect. We demonstrate the ambiguity in the electrostatic potential using several molecules of different degree of degeneracy, quasi-degeneracy, and symmetry.

PMID:
26583396
DOI:
10.1021/ct4005454

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