Subject: Semi-empirical methods + CI

From: Gerd Thys <[email-masked]>
Date: Wed, 22 Feb 1995 08:36:53 +0100 (MET)

Dear all,

I'm using PM3 and AM1 followed by MECI (as implemented in MOPAC)  to 
calculate excitation-energies of a series of (large) closed-shell organic 
molecules. 

As a test, I did some MECI calculation including 3 up to 8 orbitals to 
correct for the correlation energy (In MOPAC: MECI C.I.=3 to C.I.=8).

I was told to be careful no to add too much orbitals in de MECI-calculation, 
since part of the correction for the correlation energy is 
already included in the AM1/PM3 (and any other) parameterization. Adding 
too much orbitals would overestimate the excitation energy.

In some cases, the first singlet-singlet excitation energy at a given 
number of orbitals results in the following:

PM3:

C.I.=5   E= 3.571 eV (347 nm)
C.I.=6   E= 3.624 eV (342 nm)
C.I.=7   E= 3.472 eV (366 nm)
C.I.=8   E= 3.436 eV (361 nm)

In other words, at a given # of orbitals (in this case C.I.=6), the energy 
reaches a maximum. Normally I would expect energy to lower with adding 
orbitals (and most of the time this is the case). 

Now my question to you all: Is this 'going through a maximum' an effect 
of the overcompensation of the correlation energy? Are there any other 
effects (depending on the electronic structure of the molecule)?  

Thanks for your reaction,

Gerd

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Gerd Thys                        Ph.D. Student
Structural Chemistry Group
University of Antwerp (UIA)
Universiteitsplein 1             E-mail: [email-masked]
B-2610 wilrijk                   URL: http://www.uia.ac.be/u/thys/index.html
BELGIUM 
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