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.

As a simple yet strongly binding electron donor-acceptor(EDA) complex, BH3NH3 serves as a good example to study the electron pair donor-acceptor complexes. We employed both the ab initio valence bond (VB) and block-localized wave function(BLW) methods to explore the electron transfer from NH3 to BH3. Conventionally, EDA complexes have been described by two diabatic states: one neutral state and one ionic charge-transferred state. Ab initio VB self-consistent field(VBSCF) computations generate the energy profiles of the two diabatic states together with the adiabatic(ground) state. Our calculations evidently demonstrated that the electron transfer between NH3 and BH3 falls in the abnormal regime where the reorganization energy is less than the exoergicity of the reaction. The nature of the NH3-BH3 interaction is probed by an energy decomposition scheme based on the BLW method. We found that the variation of the charge-transfer energy with the donor-acceptor distance is insensitive to the computation levels and basis sets, but the estimation of the amount of electron transferred heavily depends on the population analysis procedures. The recent resurgence of interest in the nature of the rotation barrier in ethane prompted us to analyze the conformational change of BH3NH3, which is an isoelectronic system with ethane. We found that the preference of the staggered structure over the eclipsed structure of BH3NH3 is dominated by the Pauli exchange repulsion.

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.

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.

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).

The previously developed valence bond configuration interaction (VBCI) method (Wu, W.; Song, L.; Cao, Z.; Zhang, Q.; Shaik, S., J. Phys. Chem. A, 2002, 105, 2721) that borrows the general CI philosophy of the MO theory, is further extended in this article, and its methodological features are improved, resulting in three accurate and cost-effective procedures: (a) the effect of quadruplet excitation is incorporated using the Davidson correction, such that the new procedure reduces size consistency roblems, with due improvement in the quality of the computational results. (b) A cost-effective procedure, named VBCI(D, S), is introduced. It includes doubly excited structures for active electrons and singly excited structures for inactive pairs. The computational results of VBCI(D, S) match those of VBCISD with much less computational effort than VBCISD. (c) Finally, a second-order perturbation theory is utilized as a means of configuration selection, and lead to considerable reduction of the computational cost, with little or no loss in accuracy. Applications of the new procedures to bond energies and barriers of chemical reactions are presented and discussed.

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.

Valence bond SCF calculations have been carried out on identity proton transfers of the type X-H + X-→X- + H-X and X-H+ + X→X + X-H+ for systems where X=F-, Cl-, Br-, OH-, SH-, NH2-, PH2-,CH3-, SiH3-, OH2, NH3, and PH3. A hybrid consisting of three contributing structuressreactantlike, productlike, and ionic (X- H+ X- or X H+ X)-gives reasonable results, though additional structures afford a slight further decrease in energy. Energies somewhat below Hartree-Fock and reasonable weights are obtained. Calculated barriers agree well with those obtained by high-level ab initio calculations with correlation. Insights into the factors determining barrier height are obtained.

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.