Calculating bond orders in chemical compounds can help researchers to understand their structure and reactivity. There are several approaches to defining and calculating bond orders. However, they all have limitations, e.g., in the types of materials for which they can be used or because they are computationally expensive. The calculated bond order can also vary significantly with the quantum chemistry method used (for example, density functional theory or coupled cluster calculations).

Thomas A. Manz, New Mexico State University, Las Cruces, USA, has developed a comprehensive and efficient method to calculate bond orders for a wide range of compounds. The method uses the electron density and the spin magnetization density of the compounds as the input, which are easy to obtain from most quantum chemistry software packages. A modified Density Derived Electrostatic and Chemical (DDEC6) approach was used to partition the electrons and assign a number of electrons to each atom. The bond order is then calculated based on the delocalization of electrons between the atoms.

The method was tested extensively and performs well for many different types of materials, including non-magnetic and magnetic compounds, molecules with localized and delocalized electrons, as well as systems with covalent, metallic, and ionic bonds. The computational effort scales linearly with the number of atoms, which makes the approach feasible for large systems. In addition, the results are consistent across different quantum chemistry methods.

• Introducing DDEC6 atomic population analysis: part 3. Comprehensive method to compute bond orders,
  Thomas A. Manz,
  RSC Adv. 2017, 7, 45552–45581.
  DOI: 10.1039/c7ra07400j