Quantum chemistry, orbital description

First-row transition metals. These metals present formidable challenges for quantum chemistry. With the energies of the d orbitals being so close to those of the v orbitals for these atoms, the possibility of final states with low pole strengths cannot be ignored. In addition, the middle transition metals are generally difficult to describe with singledeterminant methods and require a more advanced approach for a proper description. 

Nano-scale and molecular-scale systems are naturally described by discrete-level models, for example eigenstates of quantum dots, molecular orbitals, or atomic orbitals. But the leads are very large (infinite) and have a continuous energy spectrum. To include the lead effects systematically, it is reasonable to start from the discrete-level representation for the whole system. It can be made by the tight-binding (TB) model, which was proposed to describe quantum systems in which the localized electronic states play an essential role, it is widely used as an alternative to the plane wave description of electrons in solids, and also as a method to calculate the electronic structure of molecules in quantum chemistry. 

Pauling always favored the Valence Bond (VB) theory over the Molecular Orbital (MO) theory for the description of the electronic structure of molecules, because the VB model resembles more the pre-quantum theoretical models of chemical bonding. However, modem quantum chemistry is dominated by MO theory, which has clearly prevailed in the computational applications. Nevertheless, a number of terms and concepts of VB theory still play an important role when it comes to the interpretation of the results of a quantum chemical calculation. 

As a foreword it must be said, perhaps constructing a too late homage to the brilliant contribution of professor Boys to Quantum Chemistry, that the first description of cartesian exponential type orbitals (CETO) was made thirty years ago by Boys and Cook [1], One can probably think this fact as a consequence of the evolution of Boys s thought on the basis set problem and to the incipient ETO-GTO dilemma, which Boys has himself stated ten years earlier [2a]. 

We now turn from the use of quantum mechanics and its description of the atom to an elementary description of molecules. Although most of the discussion of bonding in this book uses the molecular orbital approach to chemical bonding, simpler methods that provide approximate pictures of the overall shapes and polarities of molecules are also very useful. This chapter provides an overview of Lewis dot structures, valence shell electron pair repulsion (VSEPR), and related topics. The molecular orbital descriptions of some of the same molecules are presented in Chapter 5 and later chapters, but the ideas of this chapter provide a starting point for that more modem treatment. General chemistry texts include discussions of most of these topics this chapter provides a review for those who have not used them recently. 

In 2002, Nakai [24] presented a non-Bom-Oppenheimer theory of molecular structure in which molecular orbitals (MO) are used to describe the motion of individual electrons and nuclear orbitals (NO) are introduced each of which describes the motion of single nuclei. Nakai presents an ab initio Hartree-Fock theory, which is designated NO+MO/HF theory , which builds on the earlier work of Tachikawa et al. [25]. In subsequent work published in 2003, Nakai and Sodeyama [26] apply MBPT to the problem of simultaneously describing both the nuclear and electronic components of a molecular system. Their approach will be considered in some detail in this paper as a first step in the development of a literate quantum chemistry program for the simultaneous description of electronic and nuclear motion. 

The computationally viable description of electron correlation for stationary state molecular systems has been the subject of considerable research in the past two decades. A recent review1 gives a historical perspective on the developments in the field of quantum chemistry. The predominant methods for the description of electron correlation have been configuration interactions (Cl) and perturbation theory (PT) more recently, the variant of Cl involving reoptimization of the molecular orbitals [i.e., multiconfiguration self-consistent field (MCSCF)] has received much attention.1 As is reasonable to expect, neither Cl nor PT is wholly satisfactory a possible alternative is the use of cluster operators, in the electron excitations, to describe the correlation.2-3... 

The description and understanding of the nature of stereoelectronic effects is an appropriate held for the application of oiganic quantum chemistry. Molecular orbital (MO) methods " can describe the electron distribution in molecules, and the changes in internal rotation. In principle, they give the total potential energy of individual conformers completely, without the necessity to correct for various effects. Quantum chemical calculations offer a deeper insight into the orbital interactions in the molecule, and reveal the factors responsible for the stabilization of any conformation. 

The selected results just presented demonstrate the kinds of information that can be obtained by using ab initio molecular orbital and DFT calculations. The studies to date have focused for the most part on structural and energetic properties of the various atomic, ionic, and molecular species that may be involved in the thermal decomposition of energetic salts. Also, theoretical calculations have been used to obtain quantitatively descriptions of the various elementary steps postulated in mechanisms of the dissociation processes of these salts and to predict the most probable initial steps. For both ADN and AP, quantum chemistry... 

---