Molecular energy spectrum and band structure

[GJK]

Hi,

Why molecular energy spectrum and band structure giving two different results?

The band structure showing metallic character for ZGNR while the molecular energy spectrum reveals a band gap.What is the difference between the two??

What should we prefer while calculating the  HOMO-LUMO gap for a bulk; is it molecular energy spectrum or band structure?

[zh]

The band structure is usually used for a bulk configuration, while the molecular spectrum is used for a molecule configuration.  For the ZGNR, it is a periodic system and the use of molecular spectrum  can't give meaningful results.

[Anders Blom]

You can consider the molecular spectrum for bulk as the eigenvalues at Gamma. Clearly that is very different from a real band structure.