Abstract: The ytterbium chalcogenides have been studied by means of the density functional theory(DFT). The validity of the present approximate DFTformulas and the influence of relativistic effects for the lanthanide compounds have been examined by comparing the calcu-lated results with the experiments. The results show that the bond length of YbOobtained from DFTcalculations differs from the experimental value by about 0.002 nm- However,the bond energies are overestimated by DFT, even though both the gradient corrections and relativistic effects are taken into account. Introducing the exchange gradient coorrections on the basis of LDAcan reduce the calculated bond energies. Among the examined formulas for the exchange gradient correction, the formula PW86x produces a little lower energies and gives better results. The correlation correction increases the calculated bond energies. The relativistic effects make the bond lengths shorter by 0.004-0.006 nm, and make the bond energies lower by about 0. 5 eV. Analysis of the obtained results shows that the σ- and π-bonds are formed through the interaction between the 5d orbitals of Yb and the nPorbitals of the ligands. The populations in the 5d orbitals of Yb decrease through YbOto YbTe, fol-lowing the trend of the decrement of the bond energies. The relativistic effects destabilize the bonds, which may be due to the fact that the 6s electrons of Yb transfer to 5d orbitals in bond formation while the relativistic effects raise the energy of this process. The obtained dipole moments and charge distribution show that the bonds in ytterbium chalcogenides bear large covalence components which are increased by the relativistic effects.

Key words: Ytterbium chalcogenide, Density functional theory(DFT), Relativistic effects