Kizashi Yamaguchi

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...

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.

Exciton migration dynamics in a dendritic molecular aggregate

Abstract We investigate exciton migration dynamics in two types of dendritic molecular aggregate models, i.e., Bethe lattice, with mutually distinct intermolecular interaction between adjacent linear legs at the branching points. Each monomer molecule is assumed to be a dipole unit (a two-state model) coupled with each other by the dipole–dipole interaction. The generation of one exciton in the aggregates by the incident laser field and its subsequent dynamical behavior are investigated in a numerically exact manner. The two types of models show different exciton energy structures and distribution. It is also found that relaxation effects in the exciton states are essential for exciton migration from the periphery to the core.

CAS-DFT based on odd-electron density and radical density

Abstract We here propose new schemes of complete active space-density functional theory (CAS-DFT). It is based on the idea that the previous odd-electron density and radical density are used to express the spin-polarizability of CAS wavefunctions. We have applied these schemes to optimize geometries and to calculate the singlet–triplet energy gaps of several molecules and compare the results with those by the other methods and experiment. It was found that our schemes based on radical density give excellent results although our schemes drastically reduce the computational cost.

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...

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.

Mechanism of exciton migration of dendritic molecular aggregate: a master equation approach including weak exciton-phonon coupling

Abstract A master equation approach including weak exciton–phonon coupling is applied to the exciton dynamics of a dendritic molecular aggregate modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton–phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by stepwise transfer from the periphery to the core via multi-step exciton states.

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.

The gas-occlusion properties of dicarboxylate (fumarate, trans-trans-muconate and terephthalate) ruthenium(II,III) dinuclear complexes

Abstract The gas-occlusion properties of some ruthenium(II,III) dicarboxylate derivatives are reported.

Reinvestigation of the reaction of ethylene and singlet oxygen by the approximate spin projection method. Comparison with multireference coupled-cluster calculations.

We quantify a spin contamination error caused by a broken-symmetry (BS) method on the geometry at the stationary points and barrier heights of the [2 + 2] reaction between singlet oxygen and ethylene, which goes through a diradical intermediate. Several hybrid GGA, hybrid meta-GGA, and long-range corrected hybrid functionals, O3LYP, B3LYP, PBE0, MPW1B95, BHandHLYP, and omegaB97X, are examined to elucidate their original nature without the spin contamination error. For that purpose, the geometry of each reaction step for the BS state as well as its total energy is corrected by using an approximate spin projection method. The CCSD and CCSD(T) single-point calculations are also carried out at optimized geometries at the DFT level to confirm the results of the DFT methods. The single-point calculations by means of Mukherjees multireference coupled cluster with single and double excitations at CASSCF(10e,8o)-optimized geometries are also presented to assess the DFT methods. After the energy and geometry corrections, the barrier height of each functional is consistent with conventional closed-shell-type reactions even in the reaction involving singlet diradical species. We also find that the spin contamination error on the geometric change is not negligible especially at the early stage of the reaction ( approximately 3 kcal/mol), where the triplet state is the ground state.