Subject: uhf and biradicals: summary

From: [email-masked]
Date: Fri, 28 Aug 1998 10:09:44 +0200 (DFT)

Dear CCL,
 
Some weeks ago I ask about references on UHF and biradicals. I did not
receive a lot of answers. Below you could find my questions and the
answers to it.

Frederique

----------------------------------------------------------------------

Dear CCL,

I am looking for some references on calculations with unrestricted
hartree fock method on singlet biradicals in the ground state.

Thanks in advance

Frederique

----------------------------------------------------------------------

>From [email-masked] (Bernd Engels)

 Dear Frederique

 You should not do uhf computations but you should
 use DFT theory. Actually, Dr. Werner Nau from the
 Physikal Chemistry in Basel can help you, doing such calculations.
 I am in contact with him, and we exchanged some
 literature. Nevertheless, if problems appear
 I can also help you. We are able to perform
 very accurate MR-CI calculations (e.g. Angew.Chem.Int.Ed.
 1998, 37, page 1562; ibid. page 1960, JACS, 1998, 120, 6356)
 for quite large systems, so we would be able to check
 the DFT Results for model systems.

 bernd engels
 Inst. fuer Phys. und Theor. Chemie
 Universitaet Bonn
 Wegelerstr. 12
 Germany

----------------------------------------------------------------------

>From [email-masked] Fri Aug 14 23:00:43 1998

Dear Frederique,
   I don't think you can expect a UHF calculation on a singlet
diradical to
give you meaningful results.  Any hartree-fock calculation on a
singlet
state will automatically calculate a single configuration for the
closed-shell singlet.  For a singlet diradical, either closed-shell or
open-shell, you need to do at least a two-configuration calculation
such as
CASSCF(2,2).  There are some exceptions to this general rule, but they
usually involve the use of highly correlated computational methods
like
CCSD(T).  You might take a look at the following reference:

    Wierschke, Nash, and Squires,  J. Am. Chem. Soc., 1993, 115,
11958.

A general reference on diradicals is the following book:

    "Diradicals"; Weston T. Borden, Ed.; Wiley: New York, 1982,
Chapter 1.

There is also a recent paper by Schreiner describing BLYP calculations
on
singlet 1,4-dehydrobenzene (p-benzyne):

    J. Am. Chem. Soc., 1998, 120, 4184.

If you have any other questions about this, feel free to email me.
Good luck!

William

-------------------------------------------------------------------------
William L. Karney                               phone:  (415) 422-5842
Assistant Professor                             fax:    (415) 422-2346
Department of Chemistry                         email:
[email-masked]
University of San Francisco
2130 Fulton St.
San Francisco, CA  94117-1080
-------------------------------------------------------------------------

=======================================================================
Frederique BARBOSA			Institute for Organic Chemistry
					University of Basel
E-mail: [email-masked]	St. Johanns-Ring 19
phone : 061/267 11 57			4056 Basel
fax   : 061/267 11 05			Switzerland
=======================================================================