Mulliken Partial Charges

About

Mulliken partial charges are a way to estimate the distribution of electronic charge in a molecule. They are derived from Mulliken population analysis, which is based on quantum chemistry calculations using molecular orbital theory.

Key points about Mulliken partial charges:

1. Charge Distribution: Mulliken partial charges estimate how the total electronic charge is distributed among the atoms in a molecule. This helps in understanding the polarity of bonds and regions of charge accumulation or depletion within a molecule.

2. Molecular Orbitals and Electron Density: Mulliken analysis allocates electron density based on molecular orbitals, which are described as combinations of atomic orbitals. The electron density in each molecular orbital is divided among the atoms based on their contributions to the orbital.

3. Overlap Terms: A unique aspect of Mulliken analysis is how it deals with the overlap of atomic orbitals. It splits the electron density from the overlap region between atoms in a somewhat arbitrary way, which can make the results sensitive to the basis set used in the calculation.

4. Calculation Steps:

   • The electron density matrix is computed.

   • Electron density from each molecular orbital is partitioned to atoms based on their contribution.

   • The result is a set of partial charges on each atom, which sum up to the total charge of the molecule.

5. Basis Set Dependence: Mulliken partial charges are often criticized because the results can strongly depend on the choice of the basis set (the set of atomic orbitals used in the quantum chemical calculation). This can make them less reliable compared to other charge partitioning schemes like natural population analysis (NPA) or Bader charge analysis.

Applications:

• Chemical Reactivity: Mulliken charges give insight into regions of a molecule that are more electrophilic or nucleophilic.

• Intermolecular Interactions: They help in analyzing hydrogen bonding, dipole moments, and electrostatic interactions in molecular systems.

Despite their simplicity, the limitations of Mulliken charges (especially their sensitivity to the basis set) mean they are sometimes replaced by more robust methods in modern computational chemistry.

Method

The Mulliken partial charges are calcuated at the GFN2-xTB level of theory with xTB 6.6.0

Find

Under the Atomic property tree, select Mulliken Partial Charge [e]