Aromaticity is among the most frequently used concepts in the chemical literature. Nevertheless, it has to be stressed that aromaticity is not an observable that could be directly obtained from molecular wave function. So, it remains as a controversial concept and its physical origin is still being debated. This has lead to multiple ways for assessing the aromatic or anti-aromatic character of molecules. Thus, several indices of aromaticity have been proposed throughout the literature. It is well-known that aromaticity is strongly related to the electron delocalisation of the electrons along all the centres of the ring. The introduction of the n-order density matrix, and development of n-centre delocalisation indices (n-DIs), has provided a quantitative tool to measure electron delocalisation within a certain ring. Therefore, n-DIs can be regarded as a promising way to determine the actual role played by electron delocalisation in aromaticity. n-DIs are found to be in line with most
