Hidemi Nagao

Effective exchange integrals for open-shell species by density functional methods

Abstract Unrestricted Kohn-Sham (UKS) density functional theories (DFT) are applied to the calculation of potential curves and effective exchange integrals (Jab) for open-shell species. It is found that approximate spin-projected UKS DFT reproduce experimental singlet-triplet gaps for monocentric diradicals such as O, NH and CH2, and indicate an exponential decay of ¦Jab¦with increasing intermolecular distance (R) in the dimer of triplet methylene, in conformity with the size-consistency condition. However, the potential curves for the singlet state for the dimer of triplet CH2 by UKS DFT are deeper than those of ab initio second-order CI and CASPT2(D) and, therefore, the Jab value for (CH2)2 is several times larger in the region R > 3.2 A than the corresponding ab initio value. The implications of these results are discussed in relation to the necessity of the self-interaction correction to UKS DFT and the applicability of UKS DFT to molecular magnetism.

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.

A formulation and numerical approach to molecular systems by the Green function method without the Born–Oppenheimer approximation

We have proposed a new numerical scheme for the non-Born–Oppenheimer density functional calculation based upon the Green function techniques within the GW approximation for evaluating molecular properties in the full quantum mechanical treatment. We numerically calculate the physical properties of the individual motion in a hydrogen molecule and a muon molecule by means of this method and discuss the isotope effect on the properties in relation to correlation effects. It is concluded that the GW approximation is work well not only for calculation of the electronic state but also for that of nuclear state.

MODYLAS: A Highly Parallelized General-Purpose Molecular Dynamics Simulation Program for Large-Scale Systems with Long-Range Forces Calculated by Fast Multipole Method (FMM) and Highly Scalable Fine-Grained New Parallel Processing Algorithms

Our new molecular dynamics (MD) simulation program, MODYLAS, is a general-purpose program appropriate for very large physical, chemical, and biological systems. It is equipped with most standard MD techniques. Long-range forces are evaluated rigorously by the fast multipole method (FMM) without using the fast Fourier transform (FFT). Several new methods have also been developed for extremely fine-grained parallelism of the MD calculation. The virtually buffering-free methods for communications and arithmetic operations, the minimal communication latency algorithm, and the parallel bucket-relay communication algorithm for the upper-level multipole moments in the FMM realize excellent scalability. The methods for blockwise arithmetic operations avoid data reload, attaining very small cache miss rates. Benchmark tests for MODYLAS using 65 536 nodes of the K-computer showed that the overall calculation time per MD step including communications is as short as about 5 ms for a 10 million-atom system; that is, 35 ns of simulation time can be computed per day. The program enables investigations of large-scale real systems such as viruses, liposomes, assemblies of proteins and micelles, and polymers.

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.

Inverted micelle formation of cell-penetrating peptide studied by coarse-grained simulation: Importance of attractive force between cell-penetrating peptides and lipid head group

Arginine-rich peptide and Antennapedia are cell-penetrating peptides (CPPs) which have the ability to permeate plasma membrane. Deformation of the plasma membrane with CPPs is the key to understand permeation mechanism. We investigate the dynamics of CPP and the lipid bilayer membrane by coarse-grained simulation. We found that the peptide makes inverted micelle in the lipid bilayer membrane, when the attractive potential between the peptide and lipid heads is strong. The inverted micelle is formed to minimize potential energy of the peptide. For vesicle membrane, the peptide moves from the outer vesicle to the inner vesicle through the membrane. The translocation of the peptide suggests inverted micelle model as a possible mechanism of CPPs.

Modern aspects of superconductivity : theory of superconductivity

Theory of Superconductivity High-Tc Superconductivity Mechanism of Pairing Symmetry of Pairing Multiband Superconductivity Two-Gap Superconductivity Room Temperature Superconductivity Mesoscopic Superconductivity Nanosize Two-Gap Superconductivity Theory of Hybrid Ferromagnetic-Superconducting Nanosystems.

CASSCF and CASPT2 calculations of hole-doped amines with triplet carbene groups. Possibilities of high-Tc organic ferrimagnets

Abstract Ab initio CASSCF and CASPT2 calculations were carried out for hole-doped amines with triplet carbene groups such as monocations of bis(methylene) and tris(methylene) amines in order to confirm previous spin polarization (SP) and delocalization (SD) rules for ion-radicals. The ground states for the hole-doped bis(methylene) and tris(methylene) amines were calculated to be quartet and sextet, respectively. The low (LS)- and high (HS)-spin crossover occurred upon hole doping, being consistent with the SD rules. Implications of the calculated results are discussed in relation to possibilities of organic ferri- and ferro-magnets composed of CT complexes with radical substituents.

Theoretical studies on anomalous phases of photodoped systems in two-band model

Expressions of the transition temperature for various anomalous phases are first derived in the framework of the two-band model for copper oxides. The effects of the photon field for the model are discussed under the Thomas–Fermi approximation. A phase diagram for copper oxides in the photon field is shown. Phase diagrams for possible other mechanisms are also investigated theoretically in relation to electron scattering processes.

Spin-mediated superconductivity in cuprates, organic conductors and π–d conjugated systems

Abstract Previous theoretical models of organometallic magnetic conductors and superconductors are examined on the basis of ab initio Hamiltonians and Hubbard models for clusters of p–d and π–d systems in relation to the recently developed π–d systems such as (BEDT-TTF) 2 Y and (BETS) 2 Y (Y=Cu(NCS) 2 , Cu[N(CN) 2 ]X, Fe(III)X 4 (X=halogens, etc.)). The Fe(III) complexes have been used as spin sources in these systems. The phase diagrams observed for the species are similar to those of cuprate and heavy fermion superconductors because of the existence of a magnetic phase near superconducting phase. In order to elucidate the characteristic electronic structures of these species, effective exchange integrals ( J ab ) for magnetic clusters are calculated by ab initio density functional (DFT) methods. From the computational results, several model Hamiltonians such as t–J, Kondo and RKKY models are examined for a theoretical understanding of the experimental phase diagrams. Theoretical possibilities of magnetic conductors and spin-mediated superconductors are discussed on the basis of these models in the intermediate region for metal-insulator transitions. The importance of electron correlation and lattice dimensionality is emphasized in relation to high- T c superconductivity.