Bonding and noncovalent interactions. Traditionally, one must resort to molecular orbitals to understand chemical bonding. On the other hand, for noncovalent interactions, such as ionic bond and van der Waals interactions, the picture of orbitals is inadequate. NCI is well-known to identify noncovalent interactions, but it cannot describe strong covalent bonds. To resolve this issue, we proposed the SCI (strong covalent interaction) index using the Pauli energy density, which is the measure of electron repulsion due to the Pauli Exclusion Principle. Our hypothesis is that in areas where there exist strong covalent interactions, strong repulsion originated from the principle should be dominant. We introduced a dimensionless variable (r) by using the Thomas-Fermi kinetic energy density TF(r) as the reference,