From owner-chemistry@ccl.net Tue Mar 13 05:32:01 2012
From: "Frank Neese neese]|[mpi-muelheim.mpg.de" <owner-chemistry#server.ccl.net>
To: CCL
Subject: CCL: CASSCF does not produce spin densities
Message-Id: <-46470-120313045707-10917-+Py7MxAF1EMDx/g+3f8dnQ#server.ccl.net>
X-Original-From: Frank Neese <neese===mpi-muelheim.mpg.de>
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Date: Tue, 13 Mar 2012 09:56:58 +0100
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Sent to CCL by: Frank Neese [neese_._mpi-muelheim.mpg.de]
Dear Joseph,

there definitely is a spin density associated with a CASSCF wave function as long as the total spin is larger than 0. It would be a major misconception to think that spin-density only arises in a spin unrestricted framework.
I don't know about other programs, but at least the ORCA program will print spin populations following a CASSCF calculation (please note that there is a distinct difference between the spin density=a three dimensional function of space and the spin-population= are more or less well justified distribution of the spin density onto individual atoms) and also calculate expectation values such as isotropic and dipolar hyperfine couplings on the basis of the spin density. 
However, CASSCF is a Hartree-Fock level method - definitely not a high level treatment. In my humble opinion, it is unlikely to produce accurate results for iron-nitrosyl systems that are notoriously difficult. In particular, it will not do well on isotropic hyperfine couplings as these require a proper treatment of spin-polarization which you will not achieve at the CASSCF level. It only properly comes into play once you include certain classes of double excitations on top of the CASSCF reference wavefunction and also take them into account in evaluating the density. 

I hope that helps a bit?! 
Best regards,
Frank
 

Am 12.03.2012 um 18:37 schrieb Joseph Katigbak jkatigba/./stevens.edu:

> 
> Sent to CCL by: "Joseph  Katigbak" [jkatigba{:}stevens.edu]
> Hello, I am trying to determine the hyperfine coupling constants of some 
> nitrosyl compounds using a high level Multiconfigurational/Multireference 
> method like CASSCF. After running a CAS calculation using ROHF molecular 
> orbitals as guesses, I got energy convergence for my compounds. However i 
> noticed that the spin densities weren't being printed out (I was looking 
> specifically for the Isotropic Fermi contact coupling). I have been told that 
> CASSCF implements a restricted SCF colution and therefore does not generate a 
> spin density. However I know from the literature that there are several papers 
> on iron nitrosyl systems that use CASSCF and have spin densities. Is there any 
> way for me to get the spin densities/Fermi contact coupling term? or am i just 
> doing it wrong? 
> 
> 
> Thank you very much :D
> -J. Katigbak> 
> 
>