Such observations were further quantified by treating the respective charge density output files using the AIM (atoms in molecules theory) approach developed by Bader . The charge density in a chemical compound reaches a minimum between atoms thus defining a region that separates atoms from each other. The results of the charge transfer  are: (BP) = 0.30 and (AlPN) = 2.60, indicating a stronger trend from covalent to ionic character of AlP, which can be explained by the electronegativity  difference between the constituent atoms (see Introduction).