By means of the powerful Lanczos algorithm, the nonempirical valence bond (VB) model has been applied to investigate the low-lying electronic states for three typical types of homologous

By employing the Lanczos method, the effective valence bond model (EVB) is exactly solved for a series of conjugated hydrocarbons with four-membered rings. Several indices calculated from the EVB ground-state energies and wave functions are applied to interrelate with bond lengths, stabilities, and reactivities of these systems. Our results agree well with related experimental observations and deductions of molecular orbital theory. The failure of the classical valence bond theory in these systems is also exposed.

AM1 calculations are performed to study the non-linear optical (NLO) properties of push-pull quinones. The variations of molecular structure, linear (a) and NLO polarizabilities (b, c) for a set of push-pull quinones are discussed in terms of quinoid-benzenoid character (QBC), and the strength of donor-acceptor pairs The parameter already used for the polyene (dDA). dDA, bridge, is still valid in rationalizing the donor»acceptor strength for quinoid derivatives. In addition, we examined how an external electric-eld created by ìSparklesœ in the MOPAC package drives a quinoid molecule from the quinoid limit to the benzenoid limit, and how it aÜects the NLO properties. The results show that the eÜect produced by Sparkles is qualitatively similar to that of donor»acceptor pairs.

The Lanczos method is employed to solve the valence bond model exactly for n-electronic systems of benzenoid hydrocarbons. The exact valence bond ground-state wave functions have been obtained for benzenoid hydrocarbons of up to 20 n centers. By defining a set of local indices which can be easily calculated from these wave functions, bond lengths, reactivity, and local aromaticity of these benzenoid hydrocarbons have been discussed in a systematic way. The results are in excellent agreement with the known experimental facts and related predictions from molecular orbital theory.

We describe an alternative multireference coupled cluster (MRCC) method, pair-correlated coupled cluster (PCCC) theory, for treating the ground state of closed-shell systems with degeneracy or quasidegeneracy. With the separated electron pair (SEP) wave function as a reference, the PCCC wave function is truncated according to how many electron pairs are explicitly correlated. The inclusion of only two-pair correlation defines the PCCC2 scheme, and the inclusion of up to three-pair correlation gives the PCCC3 scheme. The PCCC theory is well defined within the natural orbital (of the SEP reference) description and size extensive. We present the formalism of the PCCC theory by taking the PCCC2 scheme as an example, and implement the PCCC2 and PCCC3 schemes at the ab initio level with various basis sets. Then illustrative applications are presented for systems such as the perpendicular insertion reaction path of Be into H2, the simultaneous bond stretching in AlH3 and H2O. The results show that the overall performance of PCCC methods is competitive to that of the RCCSD (T) or UCCSD (T) method at stretched geometries, but slightly inferior to that of the CCSD (T) method at the equilibrium geometry.