Subject: Summary: Semi-empirical methods + CI

From: Gerd Thys <[email-masked]>
Date: Wed, 1 Mar 1995 16:05:59 +0100 (MET)

Dear all,

Two weeks ago, I posted a question concerning Semi-empirical methods and 
CI ... Thanks to all who did reply. They were really elucidating. 

Gerd

********************* ORIGINAL POSTING ************************************

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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From [email-masked] Mar  1 15:34:52 1995
Date: Wed, 22 Feb 95 11:23:08 +0100
From: Grzegorz Bakalarski <[email-masked]>
To: [email-masked]

Dear Mr. Thys,

Maybe a key point is the version of MOPAC you use. I was told
( but I have never checked this by myself) that in version 6 (or less) of
MOPAC the C.I. routine has implemented wrong formulas ( only diagonal
elemets of matrix). I know that in MOPAC 7 (or MOPAC 93) this was
corrected.
Hope this helps.
			Grzegorz Bakalarski

P.S> Could you send me summary if you find a solution of your question ?

*****************************************************************************

From [email-masked] Mar  1 15:34:56 1995
Date: Wed, 22 Feb 1995 14:15:22 +0100 (NFT)
From: Peter Gedeck <[email-masked]>
To: Gerd Thys <[email-masked]>
Subject: Re: CCL:Semi-empirical methods + CI

Hello Gerd,

There is another aspect that needs to be considered, when talking about 
the CI-implementation in mopac. Usually the conformations used in the 
CI-calculation are ordered by there energy and then the CI-expansions are 
truncated, so that only the lowest 100 conformations are considered. 

If you look now at the type of conformations that are included in the 
CI-expansion, more and more conformations are included that are only 
single-excitations, while the number of double and higher excitations is 
reduced. The single-excitations will not contribute to the correlation 
energy of your ground state and therefore the correlation energy or the 
lowering of the ground state will be reduced. 

If you consider even more orbitals (I know this is not possible in mopac 
6 without changing the source), while keeping the number of conformations 
in your CI-calculation constant, you will find, that the S1<-S0 
absorption energy approaches the value it will have for a SCI-calculation.

I hope this will be of any help for you,

Peter

Peter Gedeck
Inst. f. Physikalische Chemie I
Egerlandstrasse 3
91058 Erlangen
Germany

Tel: ++9131 - 85 7335  Fax: ++9131 - 85 8307
E-Mail: [email-masked]
WWW: http://pctc.chemie.uni-erlangen.de/~gedeck/gedeck.html

**************************************************************************

From [email-masked] Mar  1 15:35:01 1995
Date: 22 Feb 95 17:01:43 +0100
From: [email-masked]
To: [email-masked]
Subject: Re: Semiempirical methods + CI

Dear Gerd,

If I have correctly understood, you have observed a 'going
through a maximum' effect in the EXCITATION ENERGY. If so,
then nothing unexpected is there: variation principle states
that the energy of the ground state should go down with the
increasing CI size; with some conditions, the same holds for
the first excited singlet. But nothing is known concerning
their distance.

What I said above is independent whether you use ab initio or
semiempirical theory. Do not mix over with the problem of
"overcompensation of the correlation energy" - the latter is
connected with the relation to the experimental data: if an
effect is already included in the parameters, then its
explicit inclusion may deteriorate the agreement between
theory and experiment.

Prof. I. Mayer
Budapest

************************************************************************
From [email-masked] Mar  1 15:35:07 1995
Date: Wed, 22 Feb 1995 08:51:00 -0500
From: [email-masked]
To: [email-masked]
Subject: CCL:Semi-empirical methods + CI

Remember: it is the total energy that should be lowered. Be very, very, careful
with MECI: Actual use should be C.I.=(x,y), where x is the number of orbitals 
and y is the number of singly occupied orbitals. I've seen cases where I've 
gotten a third of an electron or a fourth of an electron scattered among 
orbitals, which is a completely unphysical and spurious result and results in
higher energies. The individual orbitals movce up and down with mixing, and can 
result in transitions moving in (seemingly) odd directions, either bathochromic
or hypsochromic.

matt

****************************************************************************

From [email-masked] Mar  1 15:35:25 1995
Date: Wed, 01 Mar 1995 01:02:20 +0100 (MET)
From: Han Zuilhof <[email-masked]>
To: [email-masked]
Subject: MOPAC and CI

Dear Gerd,
In regard of your CCL question about the use of explicit CI in the calculation
of excitation properties of organic molecules, I would like to ask you
whether you would be willing to give a summary of the responses, either to
the net or to me personally. I have used PM3 with small CI's several times
quite succesfully, but almost always more in a qualitative than quantitative
sense. Not too long ago a discussion of the role of correlation energy in
semiempirical calculations was given by Tim Clark in the Israel Journal of 
Chemistry. That had -either in 1993 or 1994, and I think the latter- 
two special issues on quantum chemistry. It is in the second.
Hope this helps a bit.
Best regards,
Han

******************************************************************************
**   Dr. Han Zuilhof           **  e-mail: [email-masked]  **
**   Department of Chemistry   **  (optional: [email-masked]) **
**   University of Rochester   **                                           **
**   Rochester, NY, 14627      **  Fax:   (716) 473-6889                    **
**   USA                       **  Voice: (716) 275-2219                    **
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**                        "Excite a photochemist!"                          **
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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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