One of the landmark achievements of quantum chemistry, specifically of MO-based methods that include electron correlation, was the precise calculation of the barrier for the hydrogenexchange reaction (B. Liu, J. Chem. Phys. 1973, 58, 1925; P. Siegbahn, B. Liu, J. Chem. Phys. 1978, 68, 2457). This paper reports an accurate calculation of this barrier by two recently developed VB methods that use only the eight classical VB structures. To our knowledge, the present work is the first accurate ab initio VB barrier that matches an experimental value. Along with the accurate barrier, the VB method provides accurate bond energies and diabatic quantities that enable the barrier heightto be analyzed by the VB state correlation diagram approach, VBSCD (S. Shaik, A. Shurki, Angew. Chem. 1999, 111, 616; Angew. Chem. Int. Ed. Engl. 1999, 38, 586). This is a proof of principal that VB theory with appropriate account of dynamic electron correlation can achieve quantitative accuracy of reaction barriers, and still retain a compact and interpretable wave function. A sample of SN2 barriers and dihalogen bonding energies, which are close to CCSD (T) and G2(+) values, show that the H3 problem is not an isolated case, and while it is premature to conclude that VB theory has come of age, the occurrence of this event is clearly within sight.

Hydrogen abstraction reactions of the type X·+H-H'→X-H+H'·(X=F, Cl, Br, I) are studied by ab initio valence bond methods and the VB state correlation diagram (VBSCD) model. The reaction barriers and VB parameters of the VBSCD are computed by using the breathing orbital valence bond and valence bond configuration interaction methods. The combination of the VBSCD model and semiempiricel VB theory eads to analytical expressions for the barriers and other VB quantities that match the ab initio VB calculations fairly well. The barriers are influenced by the endo-or exothermicity of the reaction, but the fundamental factor of the barrier is the average singlet-triplet gap of the bonds that are broken or formed in the reactions. Some further approximations lead to a simple formula that expresses the barrier for nonidentity and identity hydrogen abstraction reactions as a function of the bond trengths of reactants and products. The semiempirical expressions are shown to be useful not only for the model reactions that are studied in this work, but also for other nonidentity and identity ydrogen abstraction reactions that have been studied in previous articles.

ond self-consistent field(VBSCF), breathing orbital valence bond (BOVB), and valence bond nfiguration interaction(VBCI) computations. The VB orbitals, used to construct VB functions, can be defined flexibly in the calculations depending on particular applications and focused problems, and they may be strictly localized, delocalized, or bonded-distorted (semidelocalized). The parallel version of XMVB based on MPI (Message Passing Interface) is also available.

The paper introduces a valence bond (VB) method that incorporates a polarizable continuum model of solvation, the self-consistent reaction field model. The solvation model achieves self-consistency for the charge density of the solute based on a linear combination of VB structures that interact with the reaction field of the solvent. The coupling of VB calculations with a solvent model enables one to compute the ab initio energy profiles of individual VB structures that contribute to a given state and to quantify the VB parameters of the VB state correlation diagram model in solution. Test calculations for a few systems show the validity of the method, which adds to the increasing capabilities of ab initio VB methodology.