Hi all, Currently, I'm reading the paper: Direct to indirect band gap transition in ultrathin ZnO nanowires under uniaxial compression [APPLIED PHYSICS LETTERS 94, 113114, 2009]. You can download this paper from the following url: http://h1.ripway.com/zhaohs/Direct ...ansition in ultrathin ZnO nanowires under.pdf At the top of page 3, the author said: -------------- A detailed analysis on the characteristics of atomic orbital contribution of the highest occupied molecular orbital (HOMO) shows that bonding at Gamma-point is mainly contributed from the O pz and Zn dz2 states, with equal components from all the six Zn and O in the supercell. For point E and F in Fig. 4, the key bonding characteristics are also the O pz and Zn dz2, but from only two (L=0.48 nm) or four (L=0.47 nm) Zn and O atoms. The analyses indicate that bonding of HOMO at Gamma-point is much stronger than that at E or F point. Therefore, during uniaxial compression, the energy lowering at Gamma-point will be much faster than that at E or F point. -------------- But I cann't figure out what calculations should I performed in order to obtain the above information within siesta. Furthermore, the author said that the bonding are mainly contributed from O pz and Zn dz2. But in my mind, all of the three sub-orbitals of p (px,py, pz) are exactly equivalent, and that is also the case for the five sub-orbitals of d (dz2, dxz, dxy, dx2-y2, dyz). So, how should they know the bonding are mainly contributed from O pz and Zn dz2? Could you please give me some hints? Thanks in advance. Regards.