From owner-chemistry@ccl.net Thu Jul 31 07:48:00 2008
From: "Mariusz Radon mariusz.radon-,-gmail.com" <owner-chemistry**server.ccl.net>
To: CCL
Subject: CCL:G: Locating a broken-symmetry singlet state in Gaussian
Message-Id: <-37464-080731062202-30547-xkixCz7+9YOc6dDPF7goJA**server.ccl.net>
X-Original-From: Mariusz Radon <mariusz.radon]*[gmail.com>
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Date: Thu, 31 Jul 2008 11:16:47 +0200
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Sent to CCL by: Mariusz Radon [mariusz.radon~~gmail.com]
Sue L chsue2004:yahoo.com wrote:
> Is it necessary to put a keyword guess=mix, in order to locate a
> broken-symmetry singlet state in Gaussian? If the molecule contains a
> symmetry, in which the singly occupied alpha and beta orbitals are in
> different symmetry, there is no need to put that keyword?
> 

Hi Sue,

Is it enough or not, I think, it depends on particular case... You 
always need to check whether you really have converged to a 
broken-symmetry solution or not; AFAIK, even "guess=mix" keyword doesn't 
guarantee this. So, you need to inspect the molecular orbitals and/or 
<S**2> value after the calculations are finished.

In such a case, that alpha and beta singly occupied orbitals belong to 
different symmetries, a nice way to produce the broken symmetry solution 
might be to manually permute the relevant orbitals (e.g. "guess=alter") 
and (probably) to use "scf=symm".

best wishes,
Mariusz Radon