From owner-chemistry@ccl.net Fri Oct  5 11:15:00 2012
From: "Robert W. Gora robert.gora:-:pwr.wroc.pl" <owner-chemistry.@.server.ccl.net>
To: CCL
Subject: CCL: [CCL] Resonance energy
Message-Id: <-47726-121005111431-8744-F5J5nz2XhaVy5a71LFwYFQ.@.server.ccl.net>
X-Original-From: "Robert W. Gora" <robert.gora:pwr.wroc.pl>
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Date: Fri, 05 Oct 2012 17:14:23 +0200 (CEST)
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Sent to CCL by: "Robert W. Gora" [robert.gora=-=pwr.wroc.pl]
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>> Naturally I meant Coper's CASVB being virtually identical to SCVB. Actually
>> nowadays one can use this code to perform also fully variational CASVB
>> calculations by optimizing SCVB complement which is indeed equivalent to SCVB.
>
> I am not quite sure what you are saying here. Cooper's CASVB is indeed
> equal to SCVB, but the number of structures is much less than in the
> VB2000 CASVB [...]

Dear Brian,

Thank you for the explanations and references. Regarding "variants" of CASVB
originally there were two methods proposed by Thorsteinsson et al. for orbital
transformation in order to obtain the representation of CASSCF wavefunction in
terms of SCVB wavefunction: one maximizing the overlap of \psi_{cas} and
\psi_{vb} and another one minimizing the energy functional. Now in molpro and
molcas one can perform also fully variational optimization.

For instance for benzene, CASSCF(6,6)/6-31G**, rumer basis, I obtain:

Overlap-based optimization: -230.779129404139
Energy-based optimization:  -230.779452892679
Fully variational:          -230.779551675762
CASSCF(6,6)                 -230.787092620226

Best,
Robert

--
Robert W. Góra, Theoretical Chemistry Group, ICHFiT, WUT [http://zcht.info]
[http://www.researchgate.net/profile/Robert_Gora]
[http://www.researcherid.com/rid/B-3919-2010]

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