Yasunori Yoshioka

Theoretical studies on effective spin interactions, spin alignments and macroscopic spin tunneling in polynuclear manganese and related complexes and their mesoscopic clusters

Abstract Theoretical efforts to investigate molecular magnetic materials are reviewed mainly from the viewpoint of our interest. Ab initio calculations of effective exchange interactions between spins are performed for HH, HHeH and simplified models of binuclear manganese and related complexes by using the spin unrestricted Hartree–Fock (UHF) and spin-polarized density functional (DFT), and UHF plus DFT hybrid methods. The scope and limitation of these broken-symmetry approaches are briefly discussed in relation to several computational schemes of effective exchange integrals ( J ab ). The calculated J ab values for the three systems are summarized for comparison of the computational methods. The natural orbitals (UNO or DNO) of the UHF and DFT solutions for magnetic clusters are determined by diagonalizing their first-order density matrices. They are used for MO-theoretical interpretation of superexchange interactions. The effective spin Hamiltonians such as the Heisenberg model are constructed for polynuclear complexes assuming the calculated and experimental effective exchange integrals. The macroscopic quantum tunneling (MQT) and coherence (MQC) of spins in the manganese oxide clusters are analyzed using the Heisenberg model, and the tunneling rate of spins is calculated by the coherent state path integral method. The topological rules for MQT and MQC are derived from this analysis. The path integral formulations are extended to tunneling probabilities for clusters of clusters and spin lattices with mesoscopic size. The resulting ideas are also applied to the molecular design of mesoscopic clusters of clusters in intermediate and strong correlation regimes. The active control of spins are finally discussed from the viewpoint of functionalities in molecular and biological materials, and technological applications of mesoscopic molecular magnets to quantum computing.

Density functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit (I): UBLYP, UB3LYP, and UHF calculations

Polyradicals comprised of m-phenylene-bridged organic radicals are well known as building blocks of organic ferromagnets, in which radical groups are connected with each other at the meta position in the benzene ring, and the parallel-spin configurations between radical sites are more stabilized than the antiparallel ones. Topological rules for spin alignments enable us to design organic high-spin dendrimers and polymers with the ferromagnetic ground states by linking various radical species through an m-phenylene unit. However, no systematic ab initio treatment of such spin dendrimers and magnetic polymers has been reported until now, though experimental studies on these materials have been performed extensively in the past ten years. As a first step to examine the possibilities of ferromagnetic dendrimers and polymers constructed of m-phenylene units with organic radicals, we report density functional and molecular orbital calculations of six m-phenylene biradical units with radical substituents and pol...

The effective fragment potential method. An approximate ab initio mo method for large molecules

Abstract The effective fragment potential (EFP) approximation within the ab initio MO method is proposed. Only the active electrons of a molecule are explicitly taken into account, the rest of the molecule being replaced by an effective potential. Considering, NH 3 as a two-electron system. the potential parameters have been determined and tested for various complexes.

Theoretical study of the magnetic interaction for M–O–M type metal oxides. Comparison of broken-symmetry approaches

Abstract The unrestricted Hartree–Fock (UHF) and hybrid-density functional theory (DFT) calculations have been carried out for the metal oxides such as copper oxides and nickel oxides. In order to elucidate magnetic properties of the species, the effective exchange integrals ( J ab ) have been obtained by the total energy difference between the highest and lowest spin states in several computational schemes with and without spin projection. The mixing ratios of the exchange correlation functionals in the hybrid DFT method have been reoptimized so as to reproduce the J ab values for strongly correlated oxides. The natural orbital analysis has also been performed for elucidation of symmetry and occupation numbers of the magnetic orbitals. From these calculated results, we discuss characteristics of the magnetic interactions for metal oxides in the strong correlation regime.

Interrelationships between the effective for the H3 radical

Abstract The extended Hartree—Fock wavefunction of the H 3 radical is constructed by the spin-optimized (SO) SCF procedure using the Hartree—Fock magnetically ordered set. Interrelationships among the Heisenberg model, the Hartree—Fock method and the extended Hartree—Fock method are investigated on the basis of group-theoretical considerations.

Density-functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit (II): Examination of functional dependence

As a first step toward examination of ferromagnetic polymers and dendrimers by ab initio crystal orbital methods, we elucidated candidates for monomer units with the high-spin ground states in the previous study of Part I [J. Chem. Phys. 113, 4035 (2000)] by employing density-functional (DFT) methods using Becke’s and Becke’s three parameter exchanges with Lee–Yang–Parr correlation or Hartree–Fock (HF) molecular orbital and post HF approximations. However, it was found that further computations applying other DFT functionals should be carried out to clarify the level of approximations which appropriately describe the electronic structures of magnetic molecules. In this part II, we present details of numerical results concerning magnetic properties and electronic structures for m-phenylene molecules with three neutral and one cation radicals by spin-polarized density functional methods using variety of local and nonlocal functionals and unrestricted molecular orbital methods including Mo/ller–Plesset and c...

Density functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit. III. Possibility of high-spin polymer

Because it has been well-known that the effective exchange interaction of spins is much larger within a molecule than between molecules, it is very important to design high-spin polymers linking high-spin molecules with each other as a constituent unit to obtain molecule-based ferromagnetic materials with high transition temperature. Experimental efforts to synthesize such polymers extended in one- or two-dimensions have been made in recent years while theoretical treatment of infinite polymers has been behind in the sense that no study of electronic and magnetic structure calculations by ab initio periodic approaches exists until now. We examined the magnetic properties in m-phenylene and related molecules with organic radicals by density functional and molecular orbital methods as monomer units of high-spin polymers in Part I and Part II of this series, since it has already been confirmed experimentally and theoretically that m-phenylene coupling unit leads to the ferromagnetic coupling between spins. I...

Generalized spin density functional theory for noncollinear molecular magnetism

We developed the ab initio linear combination of Gaussian type orbital program with a generalized Hartree–Fock–Slater (GHFS) functional and calculated the four hydrogen clusters as models of the noncollinear magnetic clusters. We found that the GHFS solutions with the three-dimensional noncollinear spin structure is the ground state near the Td conformation. Computational results are compared with those of the ab initio generalized Hartree–Fock functional and the differences between them are discussed. Implications of the calculated results are discussed in relation to the electronic structures of Fe4S4 and Mn4O4 clusters.

Effective exchange interactions and magnetic phase transition temperatures in Prussian blue analogs: a study by density functional theory

Abstract Unrestricted Hartree–Fock and density functional theory (UB2LYP and UB3LYP) calculations using the triple-zeta basis sets were performed in order to elucidate the signs and magnitudes of the effective exchange integrals J MM′ of models of Prussian blue analogs, (NC) 5 –M–CN–M′–(NC) 5 (M=Cr(III), V(II), M′=Cr(III), Mn(II), Ni(II), V(II) and V(III)). Having cyano groups surrounding the transition metal ions is essential for the J MM′ integrals between the transition metal ions. Magnetic phase transition temperatures T c , estimated from the calculated J MM′ values, correlate linearly with the experimental T c values.

Heisenberg models of radical reactions: Local spin (magnetic) symmetry conservations of biradical species

Abstract Interactions between a pair of two-electron (singlet or triplet) systems undergoing abstraction, insertion, exchange and association reactions are investigated on the basis of the classical Heisenberg model. Local spin-symmetry allowed and forbidden properties of these reactions are analysed in terms of the magnetic point group. It is found that the local spin-symmetry is conserved during the course of favoured reactions.