A series of diatomic molecules containing lanthanides (Ln) are studied by means of density functional calculations with or without the Ln 4f orbitals frozen. Based on analysis of the calculated results the following can be concluded. The Ln 4f orbitals do not directly participate in bonding from the view-point of traditional bonding theory, but may influence the bonding to a certain extent through mixing a little match orbitals into the localized 4f orbitals and mixing some 4f component into the delocalized molecular orbitals, causing the bond lengths shortened and the bonding energy increased by about several hundredths in general. The contribution of 4f orbitals lessens with the increase of the atomic number of lanthanides. The matches with higher electronegativity or higher valences act on the 4f orbitals more strongly, resulting in larger influence of the 4f orbitals on bonding. Except for the matches being F or O, the 4f orbitals contribute to bonding quite small for the heavy lanthanides, and they can be considered as core orbitals, but they should be considered as valence orbitals in accurate calculations for the light lanthanides. In lanthanide fluorides the 4f orbitals can be treated as core orbitals only for the heavier lanthanides near Yb, while in lanthanide oxides sensible errors may occur if the 4f orbitals are dealt with as core orbitals even for YbO.

Key words: LANTHANON, CHEMICAL BONDS, MOLECULAR ORBITAL THEORY