Valence bond theory Bonding improving

Before considering how SMIRKS can be used to carry out transformations with multiple reactants, first consider simpler unimolecular transformations. These are discussed separately because of the important use of unimolecular transformations to enforce the consistent use of SMILES throughout the database. This improves the integrity of the data in a chemical sense, rather than a relational database sense as discussed previously. The root of the issue is this There are multiple ways to represent the same molecular structure due to the limitations of valence bond theory. In valence bond theory, upon which SMILES is based, atoms have formal charges, most often zero. The bonds between atoms are shared pairs of electrons and may consist of multiple shared pairs giving rise to double, triple, or possibly even higher-order bonds between atoms. This simple theory, while quite powerful and applicable to a majority of chemical structures, leads to certain ambiguities. 

When most quantum ohemists were pursuing improvements in the molecular orbital method, he returned to the valence bond theory and developed the so-called GVB methods that allow electron correlation to be included within a valence bond framework... 

The centrality of the FNA has spawned considerable research into improvement of the approach. The strategies for obtaining better nodes are numerous. Canonical HF orbitals, Kohn-Sham orbitals from density functional theory (DFT), and natural orbitals from post-HF methods have been used. The latter do not necessarily yield better nodes than single configuration wave functions [39-41]. More success has been found with alternative wave function forms that include correlation more directly than sums of Slater determinants. These include antisymmetrized geminal power functions [42,43], valence-bond [44,45] and Pfaffian [46] forms as well as... 

Chapter 11 (Chemical Bonding II Valence Bond and Molecular Orbital Theories) has been revised to include an expanded discussion of the redistribution of electron density that occurs during bond formation, an improved introduction to Section 11-5 Molecular Orbital Theory, and an improved discussion of molecular orbital theory of the CO molecule. We have moved the section entitled Bonding in Metals online. 

The rich metric structure of macroscopic thermodynamics also presents unusually stringent tests of theoretical models. Attempts to understand thermodynamic phenomena at a molecular level seem to demand improved dynamical and quantum statistical thermodynamic models that adequately incorporate the subtleties of quantum-mechanical valency and bonding interactions. Development of such models is an active area of modem physical chemistry research, but a more complete survey of the current molecular theory of gases and liquids is beyond the scope of the present work. 

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