[Wien] band gap problem

[Jorissen Kevin]

As you may know, DFT is a ground state theory and in fact there's no theoretical justification for using excited states from a DFT calculation.  Therefore, spectra may be completely wrong - not only the band gap, but also the fine structure, ...
However, in quite a lot of cases, we see that DFT reproduces spectra quite well.  To summarize, 'we use it because it often works out'.
Band gaps, however, are often wrong (that doesn't mean trends - such as the change of a band gap due to impurities, strain, ... - are necessarily wrong, too); another famous example is the branching ratio, which can only be reproduced by more sophisticated methods (eg TDLDA).
 
 
Kevin.
 

	-----Oorspronkelijk bericht----- 
	Van: M.Y.Wu [mailto:M.Y.Wu at tnw.tudelft.nl] 
	Verzonden: wo 4/7/2004 1:23 
	Aan: wien-zeus.theochem.tuwien.ac.at 
	CC: 
	Onderwerp: [Wien] band gap problem
	
	

	Dear Wienuser,
	
	I got a question related to the band gap. From the ML, I know there is an
	underestimation for the band gap. Does it affect xspec, elnes optic
	simulation? If the material is an insulator or semiconductor from
	experiment, but the calculation get a conductor. Will the xspec, elnes and
	optic simulations from that still be reasonable or totally meaningless?
	
	
	
	Best regards,
	Mengyue
	*******************************
	Dr. Meng-Yue Wu
	Kavli institute of nanoscience Deflt
	Rotterdamseweg 137,
	2628 AL Delft, The Netherlands
	Tel:+31-15-2784796 (office)
	Fax:+31-15-2786730
	Email address: M.Y.Wu at tnw.tudelft.nl
	
	
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