Questions about MPSH calculation

[victorwu]

Hi,everyone!
  I am trying to do some MPSH calculations of two-probe system.the question is that how can I know which level is the HOMO ( or LUMO) of the super molecule in the central region.
  Should I count the valence electrons and according to Pauli principle to divide this number by two?
  To testify this, I construct a system of which the scattering region's configuration is same as that of electrode's unit cell. This is a periodic system, so the HOMO and LUMO should be near Fermi energy. But the result is Fermi energy is above the two energy level?
  Can someone give me  some detailed explanation?
  Thanks for reading! Replies would be appreciated.

[zh]

About the MPSH calculation, it has been discussed many times in this forum.  For example, see below:
http://quantumwise.com/forum/index.php?topic=185.msg1050#msg1050
http://quantumwise.com/forum/index.php?topic=361.msg2085#msg2085
http://quantumwise.com/forum/index.php?topic=456.msg2472#msg2472

Basically, when a molecule is coupled with two electrodes, i.e., in two-probe system, it will not be isolated. Due to the interaction (e.g. charge transfer) between molecule and electrodes, the energy levels spectrum of molecule in such chemical environment may be not same as the one of molecule in an isolated environment.  The understanding of so-called "HOMO" and "LUMO" levels in the MPSH spectrum can be found in the thread in the above first hyperlink.

[victorwu]

Thank you,zh！