CCL: [CCL] Resonance energy

 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]