Modern valence bond

Cooper et al. [30] were successful in rationalizing the striking variation in the S-S equihbrium bond lengths of FSSF (189.0 pm), ClSSCl (195.0 pm) and HSSH (205.5 pm) using the spin coupled (modern valence bond) theory. In the disulfur dihalides, but not for HSSH, incipient hypercoordinate character is observed at sulfur, with two partial t-like interactions in approximately perpendicular planes, and some antibonding character in the S-X (X=F or Cl) bonds. In other words, it is the form of t-like orbitals that is most rele-... 

Raimondi M, Cooper DL (1999) Ab Initio Modern Valence Bond Theory. 203 105-120... 

Raimondi M, Cooper DL (1999) Ab Initio Modern Valence Bond Theory. 203 105-120 Rao CNR, Seikh MM, Narayana C (2004) Spin-State Transition in LaCoOj and Related Materials. 234 1-21... 

A modern valence-bond description of the Diels-Alder reaction has been presented. The method of reaction classification by similarity has been expanded to include the effect of steric congestion in the classification of cycloaddition reactions. ... 

Symmetry restrictions may also be placed on the active orbitals in order to determine the nature of the resulting modern valence bond solution. This is exemplified by the common use of o - n separation for planar molecules (c/ Section 5). In earlier applications to ozone and diborane [2,4] it was also seen that the distribution of active orbitals among the irreducible representations was the deciding factor for the types of VB solution possible. It should also be borne in mind here that the nature of the lowest-lying CASSCF solution may not always coincide with that of the optimal fully-variational modem VB wavefunction. 

Modern Valence-Bond Description of the Mechanisms of Six-Electron Pericyclic Reactions... 

An overview of the CASVB approach to modern valence bond calculations 303... 

Modern valence-bond description of the mechanisms of six-electron 327... 

We focus in this Section on particular aspects relating to the direct interpretation of valence bond wavefunctions. Important features of a description in terms of modern valence bond concepts include the orbital shapes (including their overlap integrals) and estimates of the relative importance of the different structures (and modes of spin coupling) in the VB wavefunction. We address here the particular question of defining nonorthogonal weights, as well as certain aspects of spin correlation analysis. 

D. L. Thorsteinsson, J. Cooper, Gerratt, P. B. Karadakov, M. Raimondi, Theor. Chim Acta (Berlin) 93, 343 (1996). Modern Valence Bond Representations of CASSCF Wavefunctions. 

R. McWeeny, Theor. Chim. Acta 73, 115 (1988). Classical Structures in Modern Valence Bond Theory. 

M. Sironi, D. L. Copper, J. Gerratt, M. Raimondi, j. Chem. Soc. Chem. Commun. 675 (1989). The Modern Valence Bond Description of Naphthalene. 

P. C. Hiberty, S. Shaik, Theor. Chem. Acc. 108, 255 (2002). BOVB—A Modern Valence Bond Method That Includes Dynamic Correlation. 

In most modern valence bond calculations the wavefunction takes the form of a multiconfigurational expansion... 

---