Subject: Re: CCL:DFT MO energies

From: Ulrike Salzner <[email-masked]>
Date: Mon, 13 Oct 1997 07:52:40 +0300 (EET)

Dear Georg,
I have testet DFT orbital energies for some small and medium sized 
organic systems with pi-bonds (acetylene oligomers, thiophene monomer 
through trimer, pyrrole and similar systems). There are significant 
differences between LSDA and hybrid methods. The paper will soon appear 
in the Journal of Computational Chemistry.

 To summarize: The IPs (negative HOMO energies) are closer to experiment 
with hybrid functionals than at LSDA. The P86 correlation functional 
seems to work better than the LYP correlation functional. All IPs with 
DFT are smaller than experimental values (about 0.5 - 1 eV) and smaller than 
at HF with the same basis set which are quite close to experiment. In my 
limited data set it looks like there is little depenedence of the quality 
of IP on the size of the system.

 The negative LUMO energies used as EAs are by far superior with all DFT 
methods than with HF. LSDA works slightly better than hybrid  
functionals. Again the P86 correlation functional works better than LYP. 
I obtained the "experimental EAs" using experimental IPs and excitation 
energies. So they have to be treated with some caution. It looks like the 
agreement improves as the systems get larger. But the data are more than 
insecure. 

According to this, DFT corrects for the main deficiency of HF theory and 
brings the LUMO down. I reasoned that this must have to do with the error 
in the Coulomb terms in HF which seems to be absent in DFT. I was pleased 
to see that Baerends et al. came to a similar conclusion in their feature 
article in J. Phys. Chem.

 The main error in DFT seems to be due to the IP and not to the EA. Taking the 
difference between the two, I got very nice excitation energies. These 
improve as the systems get larger. The agreement is pretty poor for 
monomers but with trimers of purrole and thiophene the excitation 
energies become good.

I also tried what happens when the weight of the HF exchange is 
increased. I got the best HOMO-LUMO gaps with 30% HF exchange instead of 
20%. Geometries were not changed by this. For ethylene I increased the HF 
exchange stepwise up to 100%. There is no convergence. With 100% HF 
exchange the gap is as bad as with HF but the orbital levels are shifted 
compared to those at HF. So all this fudging is empirical and can - at 
least for now - not be put on solid theoretical ground. 

As for my personel interest in the band gaps of conducting polymers this 
is good enough. Needless to say that I had major problems to publish 
this. It took 1 1/2 years and one reviewer said it was not interesting 
enough for rapid publication.  But the first paper is in press now as 
mentioned above. There will be two more papers extending these findings 
to band gaps of polymers by extrapolation. These are submitted to J. 
Phys. Chem. and Synth. Met. Maybe in two years you can read them.

By the way. You're right I am German. I keep writing in English so more 
people can read it.

 Cheers,

Dr. Ulrike Salzner
e-mail: [email-masked]
Tel.: (312) 2664000 ext.2122

Department of Chemistry
Bilkent University
06533 Bilkent, Ankara
Turkey

On Fri, 10 Oct 1997, Georg Schreckenbach wrote:

> Dear CCL readers,
> 
> is anybody aware of papers that study DFT orbital energies?
> More precisely, I would like to learn how those orbital energies
> (Kohn-Sham eigenvalues) are different in hybrid methods like
> B3LYP as compared to GGAs like BP86 or PW91, or also as
> compared to the LDA. If there are systematic differences (which
> I am sure is the case) then I would like to know how they can
> be understood.
>     There was recently some discussion on the
> list about the possible physical meaning of the Kohn-Sham
> eigenvalues. While this is an interesting subject in itself,
> it is, this time, *not* my question.
> 
> Best regards, Georg
> 
> P.S. I will summarize to the CCL as usual.
> 
> --
> Dr. Georg Schreckenbach           Tel:     (USA)-505-667 7605
> Theoretical Chemistry T-12        FAX:     (USA)-505-665 3909
> M.S. B268, Los Alamos National      E-mail:  [email-masked]
> Laboratory, Los Alamos, New Mexico, 87545, USA
> Internet:    http://www.t12.lanl.gov/~schrecke/
> 
> 
> 
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