Magnetism has fascinated and served humanity for
almost 3000 years. Since the discovery of the lodestone
(FeO-Fe_2O_3), many different magnetic materials
have been developed, almost all based on transition
metals and/or rare-earth elements. Technological application
of magnetism also has a long history, from the compass to today's sophisticated magnetic memory systems. In contrast, the theory of magnetism has progressed more slowly, despite the efforts of great minds throughout history. The reason early theoretical models were not very valuable is now clear. Any viable theory of magnetism must be based on two inherently quantum mechanical concepts: electron spin and the Pauli exclusion principle. As such, only the 20th century has produced a competent model for magnetism,
and that model continues to evolve. Studies over the
last 20 years have revealed a bewildering array of new
magnetic phenomena that continue to challenge our
understanding of solid-state physics.

Spin control in organic molecules

Highly correlated configuration interaction (CI) wavefunctions going beyond the simple singles and doubles (CISD) model space can provide very reliable potential energy surfaces, describe electronic excited states, and yield benchmark energies and molecular properties for use in calibrating more approximate methods. Unfortunately, such wavefunctions are also notoriously difficult to evaluate due to their extreme computational demands. The dimension of a full CI procedure, which represents the exact solution of the electronic Schrodinger equation for a fixed one-particle basis set, grows factorially with the number of electrons and basis functions. For very large configuration spaces, the number of CI coupling coefficients becomes prohibitively large to store on disk; these coefficients must be evaluated as needed in a so-called direct CI procedure. Work done by several groups since 1980 has focused on using Slater determinants rather than spin ( S ^ 2 ) eigenfunctions because coupling coefficients are easier to compute with the former. We review the fundamentals of the configuration interaction method and discuss various determinant-based CI algorithms. Additionally, we consider some applications of highly correlated CI methods.

The Configuration Interaction Method: Advances in Highly Correlated Approaches

Publisher Summary This chapter discusses organic molecules with more than two coupled spins ( S ≥ 3/2). They are arbitrarily called “high-spin molecules.” Molecular solids in which more than two unpaired electrons are aligned in parallel among neighboring molecules are also discussed. The chemistry of high-spin organic molecules and highly ordered spin alignment in organic molecular assemblies are expected to open up a new field in science and are considered also to be the best area in which interesting magnetic properties from organic materials can be looked for. Ferromagnetism is found in some minerals, inorganic compounds, metals, and alloys, but has never been associated with organic compounds. The chapter establishes the molecular designs that will permit the alignment of spins in organic molecules and organic molecular solids. On the basis of rigorous molecular design guided by theory and model experiments, a number of high-spin organic molecules have been synthesized and spin alignment in molecular assemblies has been advanced with reasonable success. The chapter introduces two approaches—that is, spin alignment within a molecule and among molecules is complementary to each other in expanding our understanding of chemical bonds and organic molecules and in developing novel organic magnetic materials.

High-spin organic molecules and spin alignment in organic molecular assemblies

Physical Content, Analytical Derivation, and Rigorous Extraction of Magnetic Hamiltonians Jean Paul Malrieu,† Rosa Caballol,‡ Carmen J. Calzado, Coen de Graaf,‡,∥ and Nathalie Guiheŕy*,† †Laboratoire de Chimie et Physique Quantiques, Universite ́ de Toulouse 3, 118 route de Narbonne, 31062 Toulouse, France ‡Departament de Química Física i Inorgaǹica, Universitat Rovira i Virgili, Marcel·lí Domingo s/n, 43007 Tarragona, Spain Departamento de Química Física, Universidad de Sevilla, Profesor Garcia Gonzalez s/n, 41012 Sevilla, Spain Institucio ́ Catalana de Recerca i Estudis Avanca̧ts (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

Magnetic interactions in molecules and highly correlated materials: physical content, analytical derivation, and rigorous extraction of magnetic Hamiltonians.

L'etude demontre que les radicaux cationiques CH 3 OH +• , CH 3 NH 2 +• et CH 3 Cl +• ont des isomeres stables: • CH 2 OH 2 + , • CH 2 NH 3 + et • CH 2 ClH +

Distonic radical cations in gaseous and condensed phase

Ab initio calculations predict a singlet ground state for tetramethyleneethane

Ab initio calculations on the ring opening of cyclopropane radical cation. Trimethylene radical cation is not a stable intermediate

Ab initio calculations of the singlet-triplet energy separation in 3,4-dimethylenefuran and related diradicals

Ab initio calculations on 1,2,4,5-tetramethylenebenzene at geometries optimized by .pi.-CI calculations. Prediction of a singlet ground state for a disjoint diradical

Ping Du

Papers

Ab initio calculations predict a singlet ground state for tetramethyleneethane

Ab initio calculations on the ring opening of cyclopropane radical cation. Trimethylene radical cation is not a stable intermediate

Ab initio calculations of the singlet-triplet energy separation in 3,4-dimethylenefuran and related diradicals

Ab initio calculations on 1,2,4,5-tetramethylenebenzene at geometries optimized by .pi.-CI calculations. Prediction of a singlet ground state for a disjoint diradical

Ab initio calculations of the ring opening of methylenecyclopropane radical cation to trimethylenemethane radical cation