Enhancing the efficiency of density functionals with a novel iso-orbital indicator

The accuracy and efficiency of a density functional depends on the basic ingredients it uses and how the ingredients are built into the functional as a whole. An iso-orbital indicator based on the electron density, its gradients, and the kinetic energy density, has proven an essential dimensionless variable allowing density functionals to recognise various types of chemical bonding, both strong and weak. Density functionals constructed around the iso-orbital indicator usually require dense real-space grids for numerical implementation that deteriorate computational efficiency, compromising the improved accuracy. A novel iso-orbital indicator is proposed based on the same ingredients that retains the capability to identify the same chemical bonds while relieving the requirement of dense grids. The novel iso-orbital indicator improves recognition of electron density tail regions and is constraint-free for the exchange-correlation potential. The novel iso-orbital indicator is therefore expected to be the prime choice for density functional development and we discuss aspects of its use in cutting edge meta-generalised gradient approximations.