This brief provides an overview of theoretical research in organic ferromagnetic material design using quantum chemical approaches based on molecular orbital theory from primary Huckel to ab initio levels of theory. Most of the content describes the authors' approach to identify simple and efficient guidelines for magnetic design, which have not been described in other books. Individual chapters cover quantum chemistry methods that may be used to find hydrocarbon systems with degenerate non-bonding molecular orbitals that interact with each other, to identify high-spin-preferred systems using an analytical index that allows for simple design of high-spin systems as well as to analyze the effect of high-spin stability through orbital interactions. The extension of these methods to large systems is discussed.This book is a valuable resource for students and researchers who are interested in quantum chemistry related to magnetic property.

For design purposes one needs to relate the structure of proposed materials to their NLO (nonlinear optical) and other properties, which is a situation where theoretical approaches can be very helpful in providing suggestions for candidate systems that subsequently can be synthesized and studied experimentally. This brief describes the quantum-mechanical treatment of the response to one or more external oscillating electric fields for molecular and macroscopic, crystalline systems. To calculate NLO properties of large systems, a linear scaling generalized elongation method for the efficient and accurate calculation is introduced. The reader should be aware that this treatment is particularly feasible for complicated three-dimensional and/or delocalized systems that are intractable when applied to conventional or other linear scaling methods.