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. Could you provide us with a couple of references to CASVB as implemented in VB2000? 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]