Abstract：
In this talk, I will first introduce the density functional theory (DFT) to the audience and explain why it is possible to reduce a many-body problem to a single-particle problem in studying a many-body solid-state system. Then, I will show that the calculated single-particle Kohn-Sham energy eigenvalues agree with the spectra obtained from the angle-resolved photoemission spectroscopy (ARPES) experiments for the valence electrons in a so-called planar-like silicene structure fabricated on ZrB2(0001) but disagree with the X-ray photoemission spectroscopy (XPS) spectra for the core electrons. However, the measured core-level binding energy can be well reproduced using the many-body total energy differences obtained from the calculations adopting the same computational parameters and approximations within DFT. Finally, I will discuss the underlying reason for the findings with the audience.