Gauge-including atomic orbital correlation

Second-Order Many-Body Perturbation Theory Using Gauge-Including Atomic Orbitals. J. Gauss,/. Chem. Phys, 99, 3629 (1993). Effects of Electron Correlation in the Calculation of Nuclear Magnetic Resonance Chemical Shifts. 

This state of affairs prompted the implementation of computational methods using gauge-including atomic orbitals (GIAO) [16], first introduced by London [17]. Computer packages available nowadays [18-20] adopt London orbitals for accurate calculation of magnetic properties including electronic correlation [21]. 

In addition to the above, there are other parameters which were put forward to gauge the extent of aromaticity. These include atoms in molecules theory, generalized Polansky index, orbital effect, and polarizability. While each of these parameters measure one of the properties of the molecules in a quantitative fashion, they can be correlated to aromaticity only in conjunction with some other properties. 

---