The understanding in science implies insights from several different points of view. Alternative modern outlooks on electronic structure of atoms and molecules, all rooted in quantum mechanics, are presented in a single text. Together these complementary perspectives provide a deeper understanding of the localization of electrons and bonds, the origins of chemical interaction and reactivity behavior, the interaction between the geometric and electronic structure of molecules, etc. In the opening two parts the basic principles and techniques of the contemporary computational and conceptual quantum chemistry are presented, within both the wave-function and electron-density theories. This background material is followed by a discussion of chemical concepts, including stages of the bond-formation processes, chemical valence and bond-multiplicity indices, the hardness/softness descriptors of molecules and reactants, and general chemical reactivity/stability principles. The insights from Information Theory, the basic elements of which are briefly introduced, including the entropic origins and Orbital Communication Theory of the chemical bond, are the subject of Part IV. The importance of the non-additive (interference) information tools in exploring patterns of chemical bonds and their covalent and ionic components will be emphasized.

Chemical Reactivity in Quantum Mechanics and Information Theory introduces a thermodynamic-like description of molecular systems and provides an objective treatment of their fragments. The book formulates adequate entropic tools for probing in chemical terms and the electronic structure of molecules and rationalizing reactivity principles. It covers the information origins of chemical bonds, covalent/ionic composition, trends in molecular stability and reactivity, equilibrium polarizations and charge-transfer reconstructions of reactive complexes, as well as the phase/current promotions of molecular substrates. In addition, the book introduces a precise descriptor of molecular fragments and clarifies mostly intuitive semantics of several chemical concepts. Readers will find a precise and unbiased description of chemical reactivity phenomena in Donor-Acceptor systems in terms of quantum states and generalized concepts of Information/Communication theories.

The Quantum Information Theory (IT) perspective on entropic origins of the molecular electronic structure and chemical reactivity is examined in this book. The classical IT of Fisher and Shannon has already provided sensitive probes of the chemical bond patterns in molecules, generating tools for the bond spatial localization, formulating entropic measures of its overall multiplicity, and providing indices of the bond covalent/ionic composition. However, the amount of information carried by the particle distribution alone is not sufficient for a complete IT description of complex wavefunctions in molecular quantum mechanics, e.g., degenerate electronic states. The latter exhibit finite spatial phases and generate nonvanishing electronic currents. This monograph emphasizes the entropic chemical concepts due to the phase part of electronic states.

As well as providing a unified outlook on physics, Information Theory (IT) has numerous applications in chemistry and biology owing to its ability to provide a measure of the entropy/information contained within probability distributions and criteria of their information "distance" (similarity) and independence. Information Theory of Molecular Systems applies standard IT to classical problems in the theory of electronic structure and chemical reactivity.

The book starts by introducing the basic concepts of modern electronic structure/reactivity theory based upon the Density Functional Theory (DFT), followed by an outline of the main ideas and techniques of IT, including several illustrative applications to molecular systems. Coverage includes information origins of the chemical bond, unbiased definition of molecular fragments, adequate entropic measures of their internal (intra-fragment) and external (inter-fragment) bond-orders and valence-numbers, descriptors of their chemical reactivity, and information criteria of their similarity and independence.

Information Theory of Molecular Systems is recommended to graduate students and researchers interested in fresh ideas in the theory of electronic structure and chemical reactivity.
-Provides powerful tools for tackling both classical and new problems in the theory of the molecular electronic structure and chemical reactivity
-Introduces basic concepts of the modern electronic structure/reactivity theory based upon the Density Functional Theory (DFT)
-Outlines main ideas and techniques of Information Theory

The Information Theory (IT) is one of the youngest branches of the applied probability theory, in which the probability ideas have been introduced into the field of communication, control, and data processing. It has originated from the needs of practice, to create a theoretical model for the transmission of information, and evolved into an important chapter of the general theory of probability. An understanding of the distribution of information in molecules and its displacements accompanying chemical reactions, which involve the bond-forming and/or bond-breaking processes, is touched on in this book and provides an alternative perspective on molecular electronic structure. An insight into the entropic origins of chemical bonds and their coupling in chemical phenomena is central to many branches of chemistry.