Subject: summary: excitation energies

From: sergiusz kwasniewski <[email-masked]>
Date: Thu, 27 May 1999 18:16:24 +0200 (MET DST)

Hi all,

a couple of weeks ago I asked the following question:
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CCL:excitation energies

> Does anyone know something about the difference between the excitation
> energies calculated by RHF CIS and semi-empirical techniques like ZINDO/S
> CIS on organic structures? I've done some calculations on aromatic
> structures, but the spectra are not comparable at all! Is there a reason for
> that behaviour?

The following answers where provided to me:
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From: "Jens Spanget-Larsen" <[email-masked]>
Organization: Roskilde Universitetscenter
To: sergiusz kwasniewski <[email-masked]>
Date: Tue, 27 Apr 1999 10:20:45 +0100
Subject: Re: CCL:excitation energies
CC: [email-masked]

Dear Sergiusz,

HF CIS calculations are generally in error because of neglect of 
electronic correlation effects.  These effects tend to stabilize 
excited states more than the ground state, and the result is that HF 
CIS frequently predicts excitation energies that are much too large.  
Semiempirical procedures like ZINDO/S are especially parameterized to 
account for the effects of electron correlation (in particular, by 
empirical adjustment of the parameters used to estimate electron 
repulsion terms), and these procedures thus predict transition 
energies of the right order of magnitude, at least for the class of 
compunds and types of transition for which they were parameterized.
By the way, have you tried to apply Time Dependent Density Functional 
Theory (TD DFT) as implemented in Gaussian 98?  For example, TD B3LYP 
does a great job for electronic transitions.
Yours, Jens >--<                             
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011 
P.O.Box 260                 E-Mail: [email-masked]
DK-4000 Roskilde, Denmark   http://www.rub.ruc.dk/dis/chem/psos/
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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Hi,

for the semi-empirical techniques, I've been using the whole space (Energy
criterium is 1000 eV), while for the ab initio methods the basis set is
6-311G(d,p) within the FC approximation...

But I'm concerned about the low energy part of the spectrum, and then the
criterium inside the semi-empirical techniques doesn't change much the
appearence of the spectrum.

Did anyone encounter these inconsistencies? (More information about this,
was inside my last mail) Thanks in advance

Sergiusz

At 11:57 AM 4/26/99 -0300, Sergio Emanuel Galembeck wrote:
>Dear Sergiusz,
>    What basis are you using? I don't have experience
>on RHF/CIS, but the results depends on basis sets
>and on the choice of active space.
>    Best regards,
>                Sergio
>PS: Please, sumarize the answers.
>==============================================
>Sergio Emanuel Galembeck
>e-mail: [email-masked]
>===============================================

		     _________________________________________________

			Sergiusz Kwasniewski
			LUC SBG/TS
			Universitaire Campus Gebouw D
			3590 Diepenbeek
			BELGIUM
			tel(direct): 011/268315
			fax	   : 011/268301
			email      : [email-masked]
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