[molpro-user] how to handle singlet diradical?

[H. -J. Werner]

It is not possible to to open-shell singlets with rhf or uhf.
Only high-spin states (single Slater determinants) are possible.
Use mcscf for open-shell singlets!
Joachim Werner
On Sa, 22 Apr 2006, MHT wrote:

>I met the same problem as following, please see "OCC"
>"The total number of orbitals must be equal to (elec+spin)/2"
>does this still work for singlet diradical: _up_, _down_,
>Thanks!
>
>    * To: molpro-user at xxxxxxxxxx
>    * Subject: [molpro-user] singlet diradical
>    * From: ghoffman <ghoffman at xxxxxxxxxxxx>
>    * Date: Mon, 13 Jun 2005 10:14:09 -0400
>    * Sender: owner-molpro-user at xxxxxxxxxxxxxxxxxxxx
>
>Hello,
>
>I need some helpful advice for performing a clculation on a singlet 
>diradical.  This is a triatomic species with 16 electrons and Cv symmetry.  
>The two unpaired electrons are in orbitals of different symmetry 
>(1=totally symmetric and 2=antisymmetric with respect to reflection in the 
>symmetry plane).  Therefore, the symmetry of the electronic state is 2.  
>Because this is a singlet state then, the command for the Hartree-Fock 
>calculation ought to be:
>
>uhf;wf,16,2,0;occ,7,2;open,7.1,-2.2;
>
>The negative sign, according to the manual, designates that the spins of 
>the two unpaired electrons are opposed.  However, with this input (and 
>just about every other one I could think of), the following error message 
>is returned when the calculation crashes:
>
> SYMMETRIES FROM ORBITAL RESTRICTIONS AND WF CARD NOT CONSISTENT:   2   2  
>MS2= 0  NOPEN= 2
>
> ERROR EXIT
> CURRENT STACK:      MAIN
>
>Surely, somebody out there has performed calculations on singlet 
>diradicals.  How does one get Molpro to understand the desired electron 
>configuration?
>
>Thanks in advance.
>
>Gerry Hoffman
>
>
>
>Gerald J. Hoffman
>Assistant Professor of Chemistry
>Edinboro University of Pennsylvania
>Edinboro, PA 16412
>
>814-732-2813
>
>ghoffman at xxxxxxxxxxxx
>
>
>
>

-- 
Prof. Hans-Joachim Werner
Institute for Theoretical Chemistry
University of Stuttgart
Pfaffenwaldring 55
D-70569 Stuttgart, Germany
Tel.: (0049) 711 / 685 4400
Fax.: (0049) 711 / 685 4442
e-mail: werner at theochem.uni-stuttgart.de