CCL: [CCL] Resonance energy

 Sent to CCL by: "Salter-Duke, Brian James -" [brian.james.duke _
 gmail.com]
 On Fri, Oct 05, 2012 at 10:04:27AM +0200, Robert W. Gora
 robert.gora[#]pwr.wroc.pl wrote:
 > >>This is particularly easy in CASVB or SCVB implemented in VB2000
 (the
 > >>two methods are in fact equivalent).
 > >
 > >This is not correct. First, however, I have to point out that the term
 > >"CASVB" means different things to different people. CASVB
 implemented in
 > >MOLCAS and MOLPRO by David Cooper is indeed identical to SCVB. It is
 > >obtained by projecting out a SCVB function from a CASSCF wave function,
 > >either using an overlap criteria or an energy minimisation criteria.
 >
 > 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 = CASSCF [14 structures for SCVB and 1764 for VB2000 CASVB
 are quoted in Shaik and Hiberty's book referenced below for methane] and
 the SCVB energy is higher than that of VB2000 CASVB = CASSCF.
 Incidentally in our paper [Karadakov et al] that I referenced in my last
 post, calculations on benzene for SC(6) and on C5H5- and C7H7+ with our
 new SC approach of SCVB(6,5) and SCVB(6,7) were carried out with both
 VB2000 and MOLPRO giving identical results. We also give the
 CASSCF(6,6), CASCF(6,5) and CASSCF(6,7) energies and I think I
 reproduced all of these with VB2000 CASVB. I have certainly obtained
 identical results with VB2000 CASVB and GAMESS(US) CASSCF very many
 times.
 > Could you provide us with a couple of references to CASVB as
 > implemented in VB2000?
 There is no publication that specifically discusses the VB2000
 implementation of CASVB, but it is well described in "A Chemist's Guide
 to Valence Bond Theory by Sason Shaik and Philippe Hiberty in the
 beginning of section 9.2.3 on page 243. That section then discusses the
 Cooper CASVB, and finally it discusses the CASVB method of Hirao et al
 (Int. J. Quant.Chem., 66, 157. (1998), which is I believe essentially
 identical to the VB2000 CASVB although the VB orbitals may not be
 identical. Since the set of structures is complete, the VB orbitals are
 invarient to transformation. VB2000 on the last iteration tries to make
 them as localised as possible. VB2000 CASVB(n,m) gives the same energy
 as CASSCF(n,m).
 Brian.
 > 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]
 --
    Brian Salter-Duke (Brian Duke)   Brian.Salter-Duke]=[monash.edu
                     Adjunct Associate Professor
             Monash Institute of Pharmaceutical Sciences
       Monash University Parkville Campus, VIC 3052, Australia