Extracting covalent and ionic structures from usual delocalized wave functions: the electron-expansion methodology

The framework of the electron-expansion methodology is used for calculating the weights of local covalent and ionic structures of a chemical bond from usual delocalized wave functions, where the hole conditions are expanded in terms involving only electrons. The three-electron populations are very important for bond localization, but in Charge-Shift bonds they are not important for the covalency/ionicity of these bonds.