From owner-chemistry@ccl.net Tue Nov  8 17:38:00 2005
From: "Wai-To Chan chan!=!curl.gkcl.yorku.ca" <owner-chemistry###server.ccl.net>
To: CCL
Subject: CCL:G: transition state searches with CASSCF ???
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X-Original-From: Wai-To Chan <chan:-:curl.gkcl.yorku.ca>
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Date: Tue, 8 Nov 2005 16:22:02 -0500 (EST)
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Sent to CCL by: Wai-To Chan [chan]-[curl.gkcl.yorku.ca]
<<<<<<<
I wish to know whether it is possible to perform transition state searches at the CASSCF level of theory using Gaussian 03 rev C02. I asked the same question to the helpdesk of Gaussian but I gave up, after waiting more than 5 weeks. 
I would like to study a bi-molecular reaction in which two (neutral) molecules are transformed into two radicals (reversed dismutation reaction). 
Neither B3LYP nor X3LYP are capable to locate the TS, while unrestricted MP2 gives the correct TS but unusual high barrier (delta H activation) and delta H are obtained.

Thanks for sharing your thoughts how to handle this problem.

Kind regards,

Theo de Bruin
theodorus.de-bruin#,#ifp.fr
>>>>>>>>>>>>>>>

    If your TS is one linking between a pair of netural reactant 
molecules and two radicals and if it can be located with UHF-MP2 
then I presume it is a diradicaloid structure. If the spin-contamination 
of the UHF solution of the diradical is moderately higher than zero (  > 0.5) 
I will be surprised you can't locate the TS with DFT. Try using 
BHandHLYP starting from your UMP2 TS structure. Use a combination of 
guess=mix and stable=opt to obtain a spin-contaminated UDFT solution
and restart the transition state search. This DFT functional has a larger mix
of Hartree-Fock exchange and should stand a better chance of 
sucess than B3LYP.


  To run CASSCF you may start from the UHF solution from the UMP2 
results. Run a pop=naturalorbitals job to get the unrestricted
orbital occupancy. Then run a UNO-CASSCF job (keyword: cas(uno,n,m)) 
with guess=read specified to read off the natural orbitals from the 
checkpoint file.  For selection of the CASSCF active space 
pick the orbitals with occupancy in the range of 0.02--1.98.

  CASSCF optimization is a real pain. I will only use it as a last resort. 

Wai-To Chan