Subject: Re: CCL:G:summary: solid state/band structures

From: Ulrike Salzner <[email-masked]>
Date: Wed, 1 Oct 1997 08:22:13 +0300 (EET DST)

Dear collegues,

I read the interesting summary on polymer programs and would like to ask 
an additional question.  

There seems to be a considerable gap between the physicists and chemists 
working in this field. Following the chemical literature little meaning 
is assigned to DFT orbital energies and, to my knowledge, there is no 
physical justification do interpret them as IPs and EAs.

However, there are all these DFT band structure codes which determine 
band gaps. The band gap problem but not the fundamental question whether 
DFT eigenenergies should be used at all has been discussed. According to 
Levy, the (band) gap problem is analogous in solids and in molecules. At 
the American Physical Society Meeting in March 1997 there was a section 
on the topic and it appeared to me that DFT orbital energies have 
physical meening although the matter is by far not settled.

I am trying to obtain band gaps by extrapolating oligomer HOMO-LUMO gaps 
using DFT and run into considerable problems with reviewers (most likely 
chemists) who usually point out that DFT can not be used for this. 

As far as I understand band structure calculations, the band gaps are  
analogous to HOMO-LUMO gaps in molecules. Do I misunderstand band 
structure calculations? Is there a fundamental difference between the 
DFT eigenenergies of molecules and of solids? Any comments would be 
greatly appreciated.

Concerning the comment on ab initio codes in Georg Schreckenbach's summary: 
there is also a Hartree-Fock solid state program in Erlangen/Germany. 
This program seems to be able to do MP2 corrections. Moreover, W. 
Foerner (also formerly in Erlangen) et al. published a paper in J. Chem. 
Phys., 1997, 106, pp. 10249 on coulped cluster theory applied to polymers. 

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

Department of Chemistry
Bilkent University
06533 Bilkent, Ankara
Turkey