Qianer Zhang

XMVB: a program for ab initio nonorthogonal valence bond computations.

An ab initio nonorthogonal valence bond program, called XMVB, is described in this article. The XMVB package uses Heitler–London–Slater–Pauling (HLSP) functions as state functions, and calculations can be performed with either all independent state functions for a molecule or preferably a few selected important state functions. Both our proposed paired‐permanent–determinant approach and conventional Slater determinant expansion algorithm are implemented for the evaluation of the Hamiltonian and overlap matrix elements among VB functions. XMVB contains the capabilities of valence bond self‐consistent field (VBSCF), breathing orbital valence bond (BOVB), and valence bond configuration 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.

A practical valence bond method: a configuration interaction method approach with perturbation theoretic facility.

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 problems, 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.

GEOMETRY OPTIMIZATION OF CN (N =2?30) WITH GENETIC ALGORITHM

Abstract The geometries of C n (n=2–30) clusters have been optimized with a genetic algorithm (GA) associated with simulated annealing (SA) method on the Brenner bond-order potential energy surface (PES). It showed that the geometries are linear when n is less than 6, single ring when n is at 6–12, double ring when n is equal to 13 or 14. Multi-ring structures start as n is greater than 14. C 28 is in a fullerene structure; while C 20 and C 21 are bowl- and cap-like, respectively. The other multi-ring structures were predicted to be planar. Further analysis indicated the stability of the non-planar open structures on the Brenner PES is related to the number of the triple bonds in the clusters.

ON THE RESONANCE THEORY

Abstract In the present paper, the mathematical formulation of resonance theory is discussed. In fact, resonance theory is a representative form of the unitary group approach. Using the standard projection operator of the symmetric group, a new basis for an irreducible representation of the unitary group, called bonded tableau, can be constructed to describe a resonance structure correspondingly. The relationships between bonded tableau and Weyl tableaux and between valence bond and molecular orbital approaches are revealed. Finally, test calculations on ozone and benzene are performed.

Chemisorption of acetonitrile, pyridine and pyrazine on the Si(100)-2x1 surface: theoretical predictions

The chemisorption of acetonitrile (CH3CN), pyridine (C5H5N) and pyrazine (C4H4N2) on the Si(100)-2 × 1 surface has been investigated by means of first-principles density functional cluster model calculations. For acetonitrile, an N-end-on adsorption state and a side-on adsorption state were found, together with a transition state that connects the two adsorption states. The predicted energetics suggests that the side-on adsorption state can be readily formed at rather low temperature via the end-on precursor state. For both pyridine and pyrazine, an N-end-on adsorption state and two side-on adsorption states were revealed. In the pyridine/Si(100) chemisorption system, the primary adspecies would be the N-end-on adsorbed pyridine as the N-end-on adsorption is the most favorable and barrierless. For the pyrazine case, the N-end-on adsorbed pyrazine would be the primary adspecies at low temperature, while at elevated temperatures the primary adspecies would be the side-on adsorbed pyrazine, which is di-σ bonded onto the surface dimer through the 2 and 5 carbon atoms. In particular, the finding that the N-end-on adsorption of pyridine on the Si(100)-2 × 1 surface is substantial enlightens us as to the possibility of constructing a pyridine-based, conductive (or semiconductive) polymer film on the Si surface.

Origins of Rotational Barriers in Hydrogen Peroxide and Hydrazine.

Compared with their isoelectronic system ethane, both hydrogen peroxide and hydrazine exhibit a double well torsional energy curve where skew conformers are favored over trans conformers and cis conformers are energy-maximum states. Clearly, the involvement of the lone oxygen and nitrogen pairs, or more specifically, the enhanced stabilizing n→σ* negative hyperconjugation effect and destabilizing repulsion among lone pairs, complicates the conformational analysis. In this work, the modern ab initio valence bond (VB) method is employed to quantitatively investigate the torsional energy curves of hydrogen peroxide and hydrazine in terms of hyperconjugative stabilization, steric repulsion, and structural and electronic relaxations. It is found that if the hyperconjugation effect is completely quenched, the trans conformers will be favored, while the cis conformers are the only transition state pertaining to the torsional motion in the potential energy surfaces of H2O2 and N2H4. Although usually the steric effect includes the contributions from the electronic and geometric changes, our energy decomposition analysis shows that even the steric effect favors the skew conformers, while the electronic and geometric changes stabilize the trans conformers. Thus, we conclude that both the hyperconjugative and steric interactions lower the energy of skew conformers and eventually form low barriers from skew to trans conformers and high barriers from skew to cis conformers in both H2O2 and N2H4. Comparison between the VB and the natural bond orbital (NBO) results show similarities and discrepancies between the two methods.

Density functional study of multiple H2 adsorption and activation on a Pd6 cluster

Abstract Density functional calculations are used to determine structures and stabilities of Pd6 and hydride complexes Pd6(H)n (n=2,4). Calculations show that a triplet state of Pd6 with an elongated octahedral structure is the ground state and the Pd6 cluster can activate the second H2 molecule. Barriers of the rate-determining steps for the first H2 and the second H2 activation process are ca. 11 kcal/mol and ca. 7–9 kcal/mol, respectively. In the dihydrogen complexes, the H2-ligand behaves as donor in the molecular bonding between H2 and Pd6. Chemisorbed hydrogen atoms prefer to bridge the Pd–Pd edges in the most stable hydride complexes Pd6(H)n (n=2,4).

A quantum chemical study of the NO/MgO chemisorption system: hybrid B3LYP calculations on NO/(MgO)(n) (n = 4, 6, 8) model systems

Ab initio B3LYP cluster model calculations have been performed to explore the adsorptive behavior of NO on MgO solid. Two adsorption modes, namely the chain mode and the bridge mode, have been found for NO adsorption at low-coordinate Mg–O pair sites. NO adsorbed in the chain mode is highly activated and the resultant NO22− complex would be an intermediate in the O-exchange reaction between NO and MgO. Two possible configurations of N2O32− surface species have been found in the NO/MgO system. Our calculated IR frequencies of N2O32− surface complexes account well for the temperature dependence of the experimental IR spectra.

Density functional characterization of N2 dissociation on the step of ruthenium clusters

Mechanisms of N 2 dissociativeadsorption on small ruthenium clusters are studied by density functional calculations. The calculations indicate that the step of a ruthenium cluster has high activity for N 2 activation, where an ensemble of five Ru atoms on the stepped surface of clusters is responsible for the active site. Such high activity arises from a strong charge-transferinteraction due to local phase adaptation between the π* orbital of N 2 and the filled cluster valence orbital over the step region. Results from cluster models with different size show that the activation mechanism and the barrier are sensitive to the structural environment of the step. N 2 dissociation over the step of the 11-atom cluster is a two-step process, where the rate-determining step has a barrier of 22 kcal mol−1. N 2 dissociativeadsorption on the stepped surface of 15-atom and 21-atom clusters is a one-step process, and the barrier is ∼7–10 kcal mol−1. Theoretical calculations on the 11-atom Os and Fe cluster models reveal a general activity of the stepped sites for N 2 activation.

Study of intramolecular electron transfer with a two-state model based on the orbital deletion procedure

Carbocations H2C-bridge-CH2+ are often used as models for donor-bridge-acceptor complexes to study the role of bridges in the electron transport process. In an attempt to delineate the electron delocalization effect from the bridge to the positively charge terminal in the unrestricted Hartree–Fock (UHF) wave functions which are often used for diabatic states to compute the electronic coupling energy, we propose to employ an orbital deletion procedure (ODP) to generate the strictly localized wave functions for the initial (A) and final (B) diabatic states in the electron transfer process in the carbocations of H2C-bridge-CH2+. The electronic coupling energy VAB can be subsequently computed with the two diabatic states by solving a 2×2 secular equation. The comparison of our results with previous theoretical studies based on the widely adopted charge-localized UHF wave functions and Koopmans’ theorem in the case of positively charged 1,3-dimethylenebicyclo[1.1.1]pentane reveals that charge-localized UHF wav...