Determining Partial Atomic Charges
Despite its usefulness, the concept of a partial atomic charge is somewhat arbitrary, because it depends on the method used to delimit between one atom and the next (in reality, atoms have no clear boundaries). As a consequence, there are many methods for estimating the partial charges. According to Cramer (2002), all methods can be classified in one of four classes:
- Class I charges are those that are not determined from quantum mechanics, but from some intuitive or arbitrary approach. These approaches can be based on experimental data such as dipoles and electronegativities.
- Class II charges are derived from partitioning the molecular wave function using some arbitrary, orbital based scheme.
- Class III charges are based on a partitioning of a physical observable derived from the wave function, such as electron density.
- Class IV charges are derived from a semiempirical mapping of a precursor charge of type II or III to reproduce experimentally determined observables such as dipole moments.
The following is a detailed list of methods, partly based on Meister and Schwarz (1994).
- Population analysis of wavefunctions
- Mulliken population analysis
- Coulson's charges
- Natural charges
- CM1, CM2, CM3 charge models
- Partitioning of electron density distributions
- Bader charges (obtained from an atoms in molecules analysis)
- Density fitted atomic charges
- Hirshfeld charges
- Maslen's corrected Bader charges
- Politzer's charges
- Voronoi Deformation Density charges
- Density Derived Electrostatic and Chemical (DDEC) charges, which simultaneously reproduce the chemical states of atoms in a material and the electrostatic potential surrounding the material's electron density distribution
- Charges derived from dipole-dependent properties
- Dipole charges
- Dipole derivative charges, also called atomic polar tensor (APT) derived charges, or Born, Callen, or Szigeti effective charges
- Charges derived from electrostatic potential
- Chelp
- ChelpG (Breneman model)
- Merz-Singh-Kollman (also known as Merz-Kollman, or MK)
- Charges derived from spectroscopic data
- Charges from infrared intensities
- Charges from X-ray photoelectron spectroscopy (ESCA)
- Charges from X-ray emission spectroscopy
- Charges from X-ray absorption spectra
- Charges from ligand-field splittings
- Charges from UV-vis intensities of transition metal complexes
- Charges from other spectroscopies, such as NMR, EPR, EQR
- Charges from other experimental data
- Charges from bandgaps or dielectric constants
- Apparent charges from the piezoelectric effect
- Charges derived from adiabatic potential energy curves
- Electronegativity-based charges
- Other physicochemical data, such as equilibrium and reaction rate constants, thermochemistry, and liquid densities.
- Formal charges
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