In recent years the fundamental concepts and applied methodologies of molecular similarity analysis have experienced a revolutionary development. Motivated by the increased degree of understanding of elementary molecular properties on the levels ranging from fundamental quantum chemistry to the complex interactions of biomolecules, and aided by the spectacular progress in computer technology and access to computer power, the area has opened up to many new ideas and new approaches. This book covers topics in quantum similarity approaches, electron density shape analysis methods, and it provides better theoretical understanding of molecular similarity. Additionally, quantitative shape analysis, especially activity relations (QShAR) and the prediction of the pharmacological or toxicological effects of molecules in the related context of quantum QSAR (QQSAR). This volume written by the experts in the various subfields of molecular similarity, provides a collection of the most recent ideas, advances, and methodologies. It is the hope of the Editors that by representing these topics within a single volume, the readers will find a balanced overview of the status of the field. We also hope that the book will serve as a tool for selecting and assessing the best approach for various new types of problems of molecular similarity that may arise and it will provide a set of easy references for further studies and applications.

In the study of various phenomena in nature, the concept of similarity plays a fundamental role. Chemistry is no exception; the similarity of molecules, both in their physical properties and in their chemical reactions, provides a basis for their classification, characterization, and scientific description. Ultimately, the recognition and analysis of molecular similarities serve as the basis of an understanding of molecular structures and properties, and rep resent the first steps in the development of theoretical models explaining chemical behavior. In this role, molecular similarity is the foundation of predictive models in chemistry. Molecular similarity and molecular reactivity are strongly related. Studying the reactivities of molecules is an important tool for detecting molecular similarities and differences; alternatively, similar molecular properties often imply similar reactivities. This latter aspect is of special value, allowing chemists to make predictions concerning the outcomes of chemical reactions based on molecular similarities. In this book, the central theme, molecular similarity, is discussed from a variety of viewpoints covering the range from rigorous quantum chemical approaches to phenomenological observations expressed within appropriate physical and mathematical frameworks. The authors of the various chapters represent some of the most current fields of research on molecular similarity. It is the hope of the editor that by bringing these subjects together under the cover of this book will provide the readers with a broad perspective and a handy reference of the contemporary approaches to similarity analysis of molecules and reactions."

This volume, edited by a well-known specialist in the field of theoretical chemistry, gathers together a selection of papers on theoretical chemistry within the themes of mathematical, computational, and quantum chemistry. The authors present a rich assembly of some of the most important current research in the field of quantum chemistry in modern times. In Quantum Chemistry at the Dawn of the 21st Century, the editors aim to replicate the tradition of the fruitful Girona Workshops and Seminars, held at the University of Girona, Italy, annually for many years, which offered important scientific gatherings focusing on quantum chemistry. This volume, like the workshops, showcases a large variety of quantum chemical contributions from different points of view from some of the leading scientists in the field today. This unique volume does not pretend to provide a complete overview of quantum chemistry, but it does provide a broad set of contributions by some of the leading scientists on the field, under the expert editorship of two leaders in the field.

In recent years the fundamental concepts and applied methodologies of molecular similarity analysis have experienced a revolutionary development. Motivated by the increased degree of understanding of elementary molecular properties on the levels ranging from fundamental quantum chemistry to the complex interactions of biomolecules, and aided by the spectacular progress in computer technology and access to computer power, the area has opened up to many new ideas and new approaches. This book covers topics in quantum similarity approaches, electron density shape analysis methods, and it provides better theoretical understanding of molecular similarity. Additionally, quantitative shape analysis, especially activity relations (QShAR) and the prediction of the pharmacological or toxicological effects of molecules in the related context of quantum QSAR (QQSAR). This volume written by the experts in the various subfields of molecular similarity, provides a collection of the most recent ideas, advances, and methodologies. It is the hope of the Editors that by representing these topics within a single volume, the readers will find a balanced overview of the status of the field. We also hope that the book will serve as a tool for selecting and assessing the best approach for various new types of problems of molecular similarity that may arise and it will provide a set of easy references for further studies and applications.

In the study of various phenomena in nature, the concept of similarity plays a fundamental role. Chemistry is no exception; the similarity of molecules, both in their physical properties and in their chemical reactions, provides a basis for their classification, characterization, and scientific description. Ultimately, the recognition and analysis of molecular similarities serve as the basis of an understanding of molecular structures and properties, and rep resent the first steps in the development of theoretical models explaining chemical behavior. In this role, molecular similarity is the foundation of predictive models in chemistry. Molecular similarity and molecular reactivity are strongly related. Studying the reactivities of molecules is an important tool for detecting molecular similarities and differences; alternatively, similar molecular properties often imply similar reactivities. This latter aspect is of special value, allowing chemists to make predictions concerning the outcomes of chemical reactions based on molecular similarities. In this book, the central theme, molecular similarity, is discussed from a variety of viewpoints covering the range from rigorous quantum chemical approaches to phenomenological observations expressed within appropriate physical and mathematical frameworks. The authors of the various chapters represent some of the most current fields of research on molecular similarity. It is the hope of the editor that by bringing these subjects together under the cover of this book will provide the readers with a broad perspective and a handy reference of the contemporary approaches to similarity analysis of molecules and reactions."

This volume, edited by a well-known specialist in the field of theoretical chemistry, gathers together a selection of papers on theoretical chemistry within the themes of mathematical, computational, and quantum chemistry. The authors present a rich assembly of some of the most important current research in the field of quantum chemistry in modern times. In Quantum Chemistry at the Dawn of the 21st Century, the editors aim to replicate the tradition of the fruitful Girona Workshops and Seminars, held at the University of Girona, Italy, annually for many years, which offered important scientific gatherings focusing on quantum chemistry. This volume, like the workshops, showcases a large variety of quantum chemical contributions from different points of view from some of the leading scientists in the field today. This unique volume does not pretend to provide a complete overview of quantum chemistry, but it does provide a broad set of contributions by some of the leading scientists on the field, under the expert editorship of two leaders in the field.