Subject: Re: CCL:MCSCF / selection of configurations

From: Johannes Weber <[email-masked]>
Date: Thu, 29 May 1997 20:52:01 +0200 (MST)

Dear Roberto!

On Tue, 27 May 1997, Roberto Bianco wrote:

> 
> I would like to do a state-averaged MCSCF calculation for the first few
> electronic states of a relatively large system. 
> 
> Are there references about the possible strategies one could use to
> select the relevant configurations for my particular case ?

You didn't mention what kind of MCSCF calculation you want do and you 
didn't specify the molecule either, so my answer can't be very 
specific. In my opinion it seems to be quite difficult 
to treat 'a relative large system' with MCSCF at all (just depends 
on the word 'relative'). 

For a detailed overview of MCSCF methods you might look at
R. Shepard, Adv. Chem. Phys. _69_, 63 (1987).
 
In the CASSCF method (mostly used) you choose active molecular
orbitals (MO's) instead of configurations. If you want an overview to
CASSCF and some examples for choosing the active space (dealing with
rather small molecules) look at

B.O. Roos in 'Lecture Notes in Quantum Chemistry', 
Springer, Berlin (1992).
(There must be a review about CASSCF somewhere in a journal.) 

A more general but very instructing source for choosing configurations 
within the CI method (similar criteria as in MCSCF) is 
I. Shavitt  in Schaefer III, 'Modern Theoretical Chemistry', Vol.3, 
Chap. 6, Plenum Press, New York (1977). 

CASSCF/CASPT2 calculations of excited states on a 'larger' molecule 
(Biphenyl, C12H10) were done by
    
M. Rubio, M. Merchan, E.Orti, B.O. Roos, Chem. Phys. Lett. _234_, 
373 (1995). (Biphenyl, C12H10)

Just look in the section 'details of calculation' to find out the
authors strategies. The essence is, that MCSCF (or more precisely
CASSCF) isn't a 'black box' method, so one must combine observation
of the MO's (it's fine if you have NO's and occupation numbers) with
chemical intuition (e.g. selecting pi-MO's in aromatic compounds) and - 
last but not least - some stamina in doing
test calculations to find out the proper MO's or configurations. 

Hope it helps, Johannes.

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| Johannes Weber                     | [email-masked]   |
| Institut fuer Physikalische Chemie | fax: 0049-(0)221-470-5144     |
| Lehrstuhl Prof. Dr. G. Hohlneicher | tel: 0049-(0)221-470-4816     |
| Universitaet zu Koeln              |                               |  
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