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.

The semiempirical valence bond (VB) method, VBDFT(s), is applied to the ground states and the covalent excited states of polyenyl radicals C2n-1 H2n+1 (n=2-13). The method uses a single scalable parameter with a value that carries over from the study of the covalent excited states of polyenes (W. Wu, D. Danovich, A. Shurki, S. Shaik, J. Phys. Chem. A, 2000, 104, 8744). Whenever comparison is possible, the VB excitation energies are found to be in good accord with sophisticated molecular orbital (MO)- based methods like CASPT2. The symmetry-adapted Rumer structures are used to discuss the state-symmetry and VB constitution of the ground and excited states, and the expansion to VB determinants is used to gain insight on spin density patterns. The theory helps to understand in a coherent and lucid manner the properties of polyenyl radicals, such as the makeup of the various states, their geometries and energies, and the distribution of the unpaired electrons (the neutral solitons).

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.

d was tested by calculating the bond energies of H2, LiH, HF, HCl, F2, and Cl2 as well as the barriers of identity hydrogen abstraction reactions,X·+X'H→XH+X'·(X, X'=CH3, SiH3, GeH3, SnH3, or PbH3). It is shown that VBCIS gives results that are at par with breathing orbital valence bond method. The VBCISD method is better and its results match those of the molecular orbital based coupled cluster CCSD method. Future potential directions of the development of the VBCI approach are outlined.